After being delayed by a last-wicket partnership of 99 from Keshav Maharaj and Dane Paterson, England got over the line to comprehensively win by an innings and 53 runs in Port Elizabeth on Monday.
That put the tourists 2-1 up in the four-match series, having successfully followed up their dramatic victory at Newlands.
Ollie Pope made his first Test century, top scoring with 135 not out in England's 499-9 declared, before Dom Bess (5-51) claimed a maiden five-wicket Test haul as the Proteas were dismissed for 209 in their first innings.
Dom Sibley had starred with a century in Cape Town and Root is thrilled to see his young players thrive.
"We've had four guys under 25 making hundreds or taking five-fors and that's a great place to be," England captain Root said after taking 4-87 in South Africa's second innings.
"We want to keep continuing to provide an environment where that becomes possible and keep giving those guys that confidence to make sure when they get that opportunity they can fulfil it as they have done here.
"That's what it's about, it's not going to be the same guys every week. It's about collectively doing your role within the team. We've got a very good squad of players, there's a couple of very good players who aren't in this XI, and that's a really exciting place to be as well.
"When guys are really stepping up to the plate, especially at a young age, and showing they can perform at this level, that’s going to fill everyone with a huge amount of confidence.
"Ollie batted beautifully with Stokesy [Ben Stokes], a brilliant partnership which really put us in a commanding position and the way Bessy bowled was fantastic that first innings.
"From that point on we always felt we were in control of things. We go into a massive game next week feeling we are in a really nice spot."
The last Test of the series begins in Johannesburg on Friday and Root acknowledged England must continue to react to changing conditions.
He added: "We've adapted quickly so far. We learned our lessons very quickly from Centurion and made sure we figured out a method that was going to be successful at Cape Town and stuck to it really well. Similarly here.
"It'll be exactly the same in Joburg. It'll be very different, different challenges for both batters and bowlers.
"We must make sure we are very disciplined in how we set up and approach the conditions."
Chasing a meagre 128-7 to win in the T20 Blast Roses match at Old Trafford – Root having top scored with 32 for the visitors – Lancashire slumped to 64-5 in the eighth over.
Croft (26 not out) and Luke Wells (30) embarked upon a rebuilding job that left Dane Villas' side needing 18 from 15 deliveries, at which point Croft collapsed to the ground mid-pitch when trying to run quick single.
The batsman howled in pain and, under instruction from stand-in captain Root, Yorkshire opted not to run him out. After treatment for cramp, Croft was still in the middle when Danny Lamb hit the winning runs and Lancashire secured a quarter-final berth by four wickets.
"As a side we made a very difficult decision under pressure," Root said, as quoted by ESPNCricinfo.
"It looked very serious at first glance. In many ways it was a relief it was nothing serious. I am sure there will be many different opinions. Many people would have handled it differently."
Afterwards, Croft paid tribute to the England skipper and his team-mates, suggesting he'd have been able to have few complaints had he been run out.
"Two games in two days at 36 and a bit of sun has done me," he told Sky Sports. "I put the brakes on, they worked, and my legs just cramped up.
"I didn't know where the ball had gone. They could have taken the bails off and credit to them that they didn't."
The tourists' five-man attack shone on day five to collect their first win at The Oval since 1971, Jasprit Bumrah impressing as he became the quickest India paceman to 100 wickets in the longest format.
After Rory Burns and Haseeb Hameed shared England's first 100-run opening stand in the fourth innings since October 2016, Virat Kohli's bowlers ran riot as the hosts collapsed, losing six wickets for 47 runs.
Bumrah and Shardul Thakur, whose first-innings half-century was the fastest in Test cricket in England, provided the impetus for India and Root pointed to the former as an example.
"You look at this game, it's been frustrating today to lose 10 wickets," Root said post-match.
"But I think, when you look where the game was lost, we should have got a bigger lead first time around with the bat. It would have been nice to have another 100 runs and then we're looking at a completely different game.
"We have to be a bit more ruthless, with the first-innings runs costing us there. And with our catching, we did put down a few chances. Some were extremely difficult but when you give chances and half-chances to world-class players, they take them.
"You look at that spell from Bumrah, he recognised the key moment in the game and turned it on its head. It's something we have to look at doing ourselves."
Indeed, Bumrah has taken the second-most wickets in the series (18) behind Ollie Robinson (21), with England the 27-year-old's favourite opponents, striking 36 times at an average of 24.08.
And it was his afternoon spell, coupled with Thakur's dismissal of Root, that got the ball rolling for Kohli's men.
"I think it's something we have to get better at and keep looking to improve, recognising those key moments in games and forcing things a little bit more," Root continued.
"We did it very well at Headingley but here we did not manage that and it's something we have to be more consistent with, looking at certain situations and getting us into that advantageous position.
"It's hard to take but you have to look at how he responded from Lords to Headingley and we will look to do exactly the same at Old Trafford. We're more than capable of turning things around very quickly."
A Ben Stokes-inspired England squared the four-match series at one apiece thanks to a thrilling victory on the final session in Cape Town eight days ago.
Glory came at a price, however, with James Anderson ruled out of the remainder of the tour due to a rib problem.
It means a potential opening for Wood or Archer – both of whom are capable of frequently topping 90 mph - providing their own injury complaints are in order.
Archer sat out the previous Test due to an elbow issue, while Wood has not played competitively since England's Cricket World Cup final win over South Africa in June.
"That decision hasn't been made," Root told a news conference, with regards to Anderson's replacement.
"It's important to use [Wednesday's] session and make sure we've got a really clear understanding of where everybody is at.
"We need to find that out and make sure we're really clear going into a five-day game that they are able to stand up to the challenges that will bring, physically and mentally, having not played for a little while.
"It is the sensible thing to do to hold off, get more information on the pitch and more information on where they're at.
"If they've proven themselves to be fit, if they've jumped through every hoop and worked very hard to get back there, you have to give them that opportunity."
Chris Woakes represents an alternative if England opt not to chance Archer or Wood, but there is no doubt selecting either of the speed merchants carries added allure.
Wood's most recent Test outing saw him claim first-innings figures of 5-41 thanks to a searing spell before being named man of the match in a 232-run demolition of West Indies in St Lucia.
"With Mark, if you look at the last summer of cricket he played, he actually got through a lot of cricket injury-free which is testament to how hard he's worked on his rehab and making sure his body can cope with different things. He's been managed well by the medical staff," Root said.
"The reason we've been quite cautious in terms of rushing him back is that history. If he's 100 per cent, bowling at 90mph-plus, the skills he has at that pace are going to cause problems and will be a great asset in Test cricket."
Root confirmed a specialist spinner will feature in the England attack in Port Elizabeth, with Dom Bess expected to keep his place ahead of the uncapped Matt Parkinson after Jack Leach returned home due to illness.
England's difference-maker could once again be Stokes, who was honoured by the International Cricket Council (ICC) as its Player of the Year after a phenomenal 2019 in red and white-ball formats.
"It is hard to say any more superlatives, it is the right decision and it could not really have been anyone else," his captain added.
"The way he has performed across all the formats has been fantastic. In my opinion he is definitely the leading player in the world at the moment."
Visiting captain Root came to the crease before lunch on day one of the first Test after the tourists lost Rory Burns and Dan Lawrence, the latter for nought, with the score on 63.
Alongside opener Dom Sibley, who was trapped lbw for a typically robust 87 by Jasprit Bumrah from the final ball of the day, Root came through a stern examination before making hay as the India attack tired.
It was another sparkling display from the 30-year-old right-hander, whose 128 not out followed scores of 228 and 186 during last month's 2-0 win in Sri Lanka and left England 263-3 at stumps.
Rory Burns was back at the top of the England order having sat out the previous tour due to the birth of his first child and the Surrey left-hander looked in fine touch after Root won the toss, delightfully clipping Ravichandran Ashwin through midwicket after he and Sibley brought up the fifty partnership.
But Ashwin (1-68) had his revenge in an awful moment for Burns, who misjudged a reverse sweep to loop a simple catch to wicketkeeper Rishabh Pant and depart for 33.
Lawrence was promoted up the order after Zak Crawley was ruled out with a sprained wrist, and India skipper Virat Kohli decided to see how he might fare against quality pace bowling. He soon had an answer as Bumrah (2-40) found reverse swing to pin the Essex man in front.
Root survived a strong lbw appeal from Ishant Sharma and he and Sibley were forced to dig in against some high standard bowling either side of lunch.
Again displaying impeccable balance and judgement, Root initially put away the sweep shot that served him so well in Sri Lanka and was watchful against Ashwin and slow left-armer Shahbaz Nadeem on a surface displaying more bounce than expected.
He started to move through the repertoire when all-rounder Washington Sundar entered the attack. The partnership was 77 when England reached tea at 140-2, with Sibley through to a half-century.
They motored on in the evening session, with Root reaching three figures with a trademark tuck behind square into the leg side before shedding his earlier inhibitions and bludgeoning Ashwin for a slog-swept six. He must continue on Saturday without Sibley, whose 286-ball vigil came to a cruel end.
England had suffered an early wobble when chasing just 74 in the fourth innings of the first Test, slipping to 14-3 at one stage.
However, resuming the fifth day needing a further 36, they reached their nominal target without any further losses, Jonny Bairstow and Dan Lawrence guiding them home with an unbroken stand of 62.
Bess and Leach had laid the foundations with five-wicket hauls in the contest, as England clinched a fourth successive Test triumph overseas – their best run in the format since 1957.
"To come here with the little preparation we've had, and play in the manner that we have throughout this game, has been really impressive," Root said at the presentation ceremony.
"I thought it was a great effort yesterday (on day four). Not a lot fell for us but we worked extremely hard.
"For the two spinners to come out of the game with five-fors is a brilliant effort, especially with such little cricket that they have played coming into a series like this when the pressure is on.
"It's always tough, but without preparation it is a testament to their characters. The whole bowling group in general, to perform like we did after lunch (on day four) in particular, was a really good effort.
"It's a good start to the tour, but we know that we've got to keep getting better – and we will keep looking to do that. But I'm really pleased with the start that we've had."
Root was unsurprisingly named man of the match following his double century in England's first innings, having failed to reach three figures in Test action in 2020.
"The exciting thing, I think, is that there's still more to come from the group," he added. "The most important thing is we have that attitude during this winter tour and going into the next Test match."
Stand-in Sri Lanka captain Dinesh Chandimal, deputising in the absence of the injured Dimuth Karunaratne, admitted the hosts simply left themselves with too much to do after they were bowled out for just 135 after winning the toss on day one.
Lahiru Thirimanne made a hundred in Sri Lanka's second innings and while Kusal Perera and Angelo Mathews also weighed in with half-centuries, their spinners did not have enough runs to work with on a dry, turning pitch.
"I think it was a crucial toss to win, but the first innings cost us the game," Chandimal said. "You have to get a big total in the first innings, so that cost us on this kind of track.
"Credit to Thirimanne, Angelo, Kusal, everyone did their bit as a batting unit in the second innings. Another 70-80 runs it could have been a different story."
Sri Lanka will have the chance to draw level in the second and final Test of the series, which begins on Friday at the same venue.
The England and Wales Cricket Board (ECB) announced on Tuesday, that subject to government approval, England will contest three matches with the Windies at the Ageas Bowl and Old Trafford.
Root's men were set to face the Windies in a three-Test series beginning on Thursday with matches at The Oval, Edgbaston and Lord's but that was not possible due to the coronavirus pandemic.
The England captain may be absent for the first Test of the re-arranged series at the Ageas Bowl, starting on July 8, with the ECB exploring ways for him to leave the bio-secure bubble that will see players isolated from the general public.
Root said: "The start of July is the due date, so that complicates things slightly.
"In terms of the bubble, and the pregnancy, it's being discussed with the medical team. At the minute, it's still open for discussion, how that will finally look, I'm not exactly sure. It will have to come down to government advice."
Root confirmed he would be at the birth even if it required him missing the first Test and said of prospective deputy Ben Stokes: "I think if Ben was captain he would be fantastic.
"One of his great qualities as a leader is he sets the example. He drags people with him and gets the best out of the players around him.
"He'll have people wanting to play for him and short-term he'd be a huge success."
England started a new era with Stokes as captain and Brendon McCullum as head coach by chasing down a target of 277 at Lord's on day four to take a 1-0 lead in the three-match series.
The prolific Root became only the second England batter to score 10,000 runs in the longest format with a majestic unbeaten 115, combining with the excellent Ben Foakes (32 not out) for an unbeaten stand of 120 under grey skies in London.
Root is the 14th player to reach that milestone and got there at exactly the same age - 31 years and 157 days - as his former team-mate Alastair Cook.
Man of the match Root was untroubled as he made a 26th Test hundred in his first international since stepping down as skipper.
Root was delighted to step up for his former vice-captain Stokes, who made an important half-century on Saturday, and seal England's first win in 10 matches in the longest format.
He said during the post-match presentation: "The number of times Ben has won us Tests under my leadership, it's a great chance for me to give back to him. I love batting.
"I want to score as many runs as I can and win us as many Test matches as I can. As long as I've got the energy and the drive to do it, I'll do it. I couldn't wish for a better person to be leading this team."
He added: "It feels fantastic, more than anything for us to have won this Test match after such a long time. It's been so enjoyable.
"Hopefully we can use this as a way to step forward."
Stokes saluted Root for ensuring his reign got off to a dream start.
"Scoring a hundred and 10,000 runs - what a player, what a man," he said.
Stokes knows England remain a work in progress after they edged in front in the three-match series.
He added: "It was a great Test match. It always seems to be, England v New Zealand - especially at Lord's. Regardless of being captain, the first Test of the summer always has a special buzz about it.
"It was a special week with all the build-up and to come away with the win makes it even better. We're happy leaving here with a win but one thing that everyone needs to take into account is that it's a change of mindset for everyone and it's not something that's going to happen overnight."
Resuming on 66, the England captain batted throughout truncated day two in masterful fashion, reaching 168 at tea before rain and bad light put paid to the final session, with the tourists on 320-4 - a first-innings lead of 185.
Debutant Dan Lawrence made a wonderfully fluent 73 as part of a 173 alongside Root, with Sri Lanka's main hopes of avoiding a heavy defeat already seemingly pinned on further unseasonably wet weather.
Events might have panned out very differently had Dilruwan Perera removed Root lbw with the first ball of the day, but an umpire's call fell in the batsman's favour and he was off and running with a straight four crunched back down the ground in the same over.
The tireless Lasith Embuldeniya claimed his third wicket of the match as Kusal Mendis held a sharp chance at gully to dismiss Jonny Bairstow three runs shy of a half-century.
Unfortunately for Sri Lanka, Lawrence showed no sign of debut nerves as his attractive, smooth-wristed style transferred easily to the Test arena.
The highlight of the 23-year-old's 150-ball knock came when he launched left-arm spinner Embuldeniya for a huge, slog-swept six.
Root and Lawrence played in unhurried fashion off front and back foot, devouring a heavy diet of spin bowling until Dilruwan found extra bounce with the second new ball to have the youngster caught by Mendis at short leg.
Jos Buttler's scratchy progress against Dilruwan showed batting might not be quite as easy on a turning surface as Root and Lawrence suggested, meaning England are ominously placed despite the lost time.
Root begins pivotal year in emphatic fashion
A packed 2021 schedule is likely to define Root's reign as England's Test captain, with home and away series against India preceding a trip to Australia and a third crack at Ashes glory in his tenure.
He probably took a light lunch, having ended the first session on 99 in pursuit of a first ton in the longest format since November 2019.
In truth, the immaculate balance and judgement Root displayed throughout made an 18th Test century feel like a formality and, after nudging a single off Dilruwan from the second ball of the session, he went on to surpass Kevin Pietersen's 151 in Colombo in 2012 to make England's highest individual score in Sri Lanka.
Lawrence gives England a handy problem
Root and head coach Chris Silverwood will need to call on squad depth as they negotiate the challenges of the year to come and Lawrence's stylish introduction to the middle order was most welcome.
A remodelled Test side designed to bat time at the top of the order with the likes of Dom Sibley, Zak Crawley and Rory Burns now has another option in the engine room to make the purists purr. Ollie Pope knows he must hit the ground running on his expected return from shoulder surgery in India.
Embuldeniya gamely holds the fort in Galle
Sri Lanka's spinners have matched their batsmen for ineptitude at times, with Dilruwan struggling for length before finding some rhythm prior to tea and leg-spinner Hasaranga De Silva (0-63) going at more than four an over.
Skipper Dinesh Chandimal has turned time and again to Embuldeniya, who has wheeled through 38 overs for his 3-131. If a five-wicket haul arrives it will be the result of relentless toil.
The former Test captain has eschewed red-ball cricket with Yorkshire in order to play a debut season in the white-ball Indian Premier League.
In a Rajasthan Royals side packed with talent such as fellow England man Jos Buttler and India's Ravichandran Ashwin, though, he is yet to play a game.
Root is unconcerned, however, seeing his time in the IPL as key to broadening his all-round game before two major contests later this year.
"When I was England captain, I didn't feel it was fair on the rest of the team for me to go and play, given what the [Test captaincy] role needed and what it deserved," he told Sky Sports News.
"Now I feel like there are more opportunities to do that without the responsibility, and I'm just trying to look to keep developing as a player. I love county cricket and I think it is so important for the English game.
"[But] looking ahead to the rest of the year, the chance to play white ball cricket out in India, ahead of a World Cup for England in these conditions as well, I think will benefit me hugely.
"You look at 2019 and how incredible that was with a World Cup and [The] Ashes. There are so many opportunities for players to do wonderful things and create unbelievable memories.
"They are the things that you play for, to be a part of some special memories and cool experiences and they certainly all lay ahead for us."
England will face Australia for the first time since Root stepped down from the captaincy, with successor Ben Stokes overseeing a dramatic turnaround in fortunes.
They will then return to India for the latest edition of the ICC Cricket World Cup, which they famously won in a dramatic Super Over finale against New Zealand in 2019.
England levelled the five-match series at 1-1 with a resounding innings victory at Headingley last weekend after bowling their opponents out for only 78 on the first day.
India struggled with the bat again after being put in by Root at The Oval on Thursday, captain Virat Kohli falling for 50 as they collapsed to 127-7.
The recalled Shardul Thakur (57) blasted the fastest Test half-century ever made in England, reaching the landmark off just 31 balls, before India were all out in the 62nd over after the final three wickets fell for one run.
Jasprit Bumrah (2-15) dismissed Rory Burns and Haseeb Hameed to reduce England to 6-2 in a brilliant opening spell and Umesh Yadav struck a huge blow by bowling the in-form Root (21).
England reached 53-3 at stumps - trailing by 138 - with Dawid Malan unbeaten on 26 and nightwatchman Craig Overton (one not out) ensuring no further damage was done.
Woakes took 4-55 in his first Test for just over a year, including seeing the back of Rohit Sharma with a peach of a delivery in his first over.
He also dismissed Ravindra Jadeja, promoted to number five in the order, and was gifted a further wicket when Rishabh Pant was caught at mid-off trying to hit over the top. Thakur was his final victim, though not before playing a potentially crucial cameo.
Kohli showed his class with some glorious drives before he edged a great delivery from Ollie Robinson (3-38) through to Jonny Bairstow, who took four catches on his return to wicketkeeping duties with Jos Buttler absent.
James Anderson and Overton also took a wicket apiece in another relentless performance from the England attack.
Bumrah was run out by Burns without facing a ball in a chaotic end to the India innings, but Kohli's men hit back impressively in the final session.
WOAKES MAKES UP FOR LOST TIME
Woakes had not played for his country in the longest format since last August and he made up for lost time having recovered from a heel injury.
Pant's latest rush of blood to the head gifted Woakes a third wicket and although he took some treatment from Thakur, the seamer had the last laugh when he ended his entertaining knock thanks to a well-judged review.
THAKUR TAKES HIS CHANCE
Thakur - recalled along with Umesh as Mohammed Shami and Ishant Sharma missed out - was another to make his mark, producing a swashbuckling knock after striding to the crease with his team in big trouble.
He was dismissed without scoring in the first match at Test Bridge but went on the attack to give India some hope as he dominated a much-needed eighth-wicket stand of 63 in just eight overs, hitting three sixes and seven fours in a dramatic hour at the crease, during which he faced only 36 balls.
England became the first visiting team to win three matches in a Test series in Pakistan this week, taking them to nine victories from 10 Tests since Stokes replaced Root as skipper in April.
Prior to Root's decision to step down, England suffered a demoralising 1-0 series loss to West Indies and were thrashed 4-0 by Australia in the Ashes.
With England now earning plaudits for their fearless style under coach Brendon McCullum and captain Stokes, Root has no regrets concerning the end of his time at the helm.
"We weren't performing," Root recalled in an interview with Sky Sports. "We weren't delivering what I thought we were capable of, and it had a massive impact on me away from cricket as well.
"If you can't even be yourself, then you aren't able to give what that role requires, lead in the right way or lead a group of players.
"For a number of reasons, it was the right thing to do, to step back. It needs someone to throw absolutely everything at it.
"As soon as I made that decision, it was quite clear that it was the right one, and I've not looked back. I gave everything to that role and threw everything at it. I am really proud of the way I went about that."
England's aggressive approach was on display as they posted their fifth-highest innings total in history (657) in their Pakistan opener, and Root feels the team's enjoyment of their new style has paid dividends.
"Brendon's got a very aggressive approach to how to play the game, I think Ben is even more aggressive! It is exciting," Root said.
"When you are 10, 12, or even younger, you're playing in the back garden – that's the feeling that you get – that raw enjoyment of smacking the ball, bowling fast, or spinning it miles. That's something you should never really lose as a player.
"It's easy to forget, in the hype of the pressurised environments and demands of constantly putting in performances and winning. But it is so important that you enjoy what you do, and you're more likely to get the best out of yourselves and it's something that we've paid quite a lot of attention to."
England will attempt to reclaim the Ashes on home soil in 2023, with the series set to begin at Edgbaston on June 16, and Root is enthusiastic about their prospects.
"Everything ramps up. The interest around Test cricket in this country goes up a notch," Root said of playing in a home Ashes series.
"It's a great opportunity for us, and with how we played our cricket under Ben last summer, it's a very exciting place to be with a big series like that coming around.
"We've proven to ourselves we can do it against two of the best teams in the world in our own conditions, so it will be great to take Australia on and take that one step further again."
England's Test captain had failed to reach three figures in the format during the previous year, yet he put that lean spell behind him to deliver a double hundred in the series opener against Sri Lanka.
The Yorkshireman made 228 - the second highest Test score recorded by an England player in Asia - to help his team seize control of the first Test in Galle, though the home side hit back when left to deal with a first-innings deficit of 286.
While half-centuries from Kusal Perera and Lahiru Thirimanne, who ended day three on 76 not out, held up the tourists' push for victory, Sri Lanka still have plenty to do with the bat.
For Root, though, the hard work has already paid off, as he explained the key was finding the right rhythm again at the crease.
"I've changed quite a few things, to be honest," Root told the host broadcasters in Sri Lanka at the conclusion of play on day three.
"I've tried to get a bit more rhythm back into my batting, just trying to find triggers and things I can do when the bowler is running in, or at the end of his mark, which might just give me a sort of rhythm to things.
"It looks a bit different to previously, but it feels really good."
Root has now managed four Test double centuries - only Wally Hammond and Alastair Cook have managed more for England - while his 321-ball knock in Galle saw him pass a notable personal milestone.
The 30-year-old became the seventh England batsman to reach 8,000 Test runs; he could rise several spots on the all-time list during the coming months too, as a hectic schedule provides plenty of opportunities to add to his career tally.
After completing their two-Test trip to Sri Lanka, England have home and away series against India to come, while the end of 2021 sees an Ashes tour to Australia.
The hectic schedule comes after 2020 was disrupted due to the coronavirus pandemic, though Root made use of his time by analysing not just his own game but also the methods used by other players, including New Zealand captain Kane Williamson, now ranked as the best Test batsman in the world.
"There was so much time to look back at things and watch other cricket from around the world, some of the best players," Root said about the impact of the COVID-19 health crisis.
"Kane at the minute is playing unbelievably well, so [I was] watching him and how he goes about things. You've got to learn off your peers - I think it's important to do that - and try and advance your game as much as possible.
"We've got some fantastic coaches as well. [I'm] Trying to tap into the likes of Jacques Kallis, Paul Collingwood and Graham Thorpe on a regular basis.
"I'm very fortunate to have those guys to learn from and work with."
The Yorkshire and England batter succeeded Alastair Cook as red-ball captain in 2017 and resigned last April after a torrid run of one win in 17 Tests, with a 1-0 series defeat to West Indies the final straw.
Root still delivered remarkable returns with the bat in 2021, despite England's struggles, scoring 1,708 runs in 15 matches, including two double centuries and a further four tons.
A new Ben Stokes and Brendon McCullum era has somewhat transformed England's red-ball cricket, with two convincing series triumphs over New Zealand and South Africa either side of victory over India to secure a draw in the rescheduled meeting.
Root has still played a vital role, scoring three Test centuries since stepping down as captain, though he admitted the change was necessary after the role had started to impact him.
"The captaincy was starting to take a toll on me," Root told the Mail on Sunday.
"The limited time I did get to spend with family, which should be enjoyed and treasured, I wasn't able to do that. I wasn't really there. I came to realise that that had been the case for a little while.
"There were times when I was thinking about something I couldn't control or something that hadn't happened previously. You go in on yourself.
"We would still do what we would normally do as a family, but I would not be listening. I just felt like a bit of a zombie almost.
"I could start seeing it have an impact on me as a person. You want to bring your personality to the role, not bring the role to your personality. It was reversing into something slightly unhealthy."
Root will hope to continue to support Stokes' England as McCullum's side head to Pakistan for a three-Test series, which starts on December 1 at Rawalpindi Cricket Stadium.
Root succeeded Alastair Cook as red-ball captain in 2017 and resigned last April after a torrid run of one win in 17 Tests, with a 1-0 series defeat to West Indies the final straw.
Yorkshireman Root still delivered remarkable returns with the bat in 2021, despite England's struggles, scoring 1,708 runs in 15 matches, including two double centuries and a further four tons.
Only Pakistan's Mohammad Yousuf (1,788 in 2006) and West Indies legend Viv Richards (1,710 in 1976) have ever managed more in a single calendar year.
Key, tasked with transforming English cricket in his new role, appointed Ben Stokes as the new skipper of the Test side but was quick to credit Root for his efforts during a tumultuous period.
"I remember just thinking: Oh, my God. Joe Root. How well has he done?," Key told Sky Sports. "We obviously know he's had a great year but he has had that year when he was doing everything as well it seemed.
"There was so much put on his shoulders. He was trying to be the ambassador for the England team that he is as a captain, and in a team that was struggling and the way that he was playing and what he was able to do. It's honestly one of the great achievements.
"Statistically it will just look in history as 'He got this amount of runs and he did this and he was this as a captain', but it won't actually say in there, 'Oh, and by the way, it was the most horrendous time to be an England cricketer, where you are under so much pressure and the captain was the lone man doing it all'.
"It's one of the great sporting achievements. And as well for him to then now seem to have somehow parked it all to some degree and he's like, right, what do you need? How do we move on from here? How can I help Ben? We probably don't realise how lucky we are to have him."
James Anderson and Stuart Broad have 1,177 Test wickets between them but were surprise omissions for the tour of West Indies, as Root travelled with a new-look side under the interim stewardship of Paul Collingwood.
However, Key and Stokes have both made it clear that England plan to reintroduce the pair for the three-Test series against New Zealand, which starts on June 2 at Lord's.
Key also acknowledged why Broad and Anderson were left out of the squad, even if he would not have done the same thing.
"I don't look at everything as just black or white, I always look at the reason," Key said. "I always thought this as a pundit; if there's logic behind what they're doing to some degree, then fine, I can understand why people are doing it.
"There was no logic to me when England in the World Cup in 2015, dropped Alastair Cook and then brought in Gary Ballance. There's no part of me that can understand why you would do that.
"But there's part of me that can understand with Broad and Anderson away from home, getting on a bit as well when you wanted to try and find out about other people. I can understand that. It doesn't mean that I would have done it – but I've shown now that they've come back."
Anderson claimed his 400th Test wicket on home soil while Ollie Robinson sealed a second five-for of his England career as the hosts cruised to victory by an innings and 76 runs in Leeds.
In total, India lost eight wickets for 63 runs on Saturday, having headed into day four on 215-2 and looking in a good position to at least force England to bat again.
The series is now level at 1-1 going into the last two Tests, with the Oval next up.
With Jofra Archer and Stuart Broad missing from the attack, Anderson finished with figures of 4-69 at Headingley, and Root hailed the paceman as the greatest of all time.
"It's what he does, why he's the GOAT of Tests," said Root, who scored 121 on his home ground, at the post-match presentation.
"He sets a wonderful tone for the bowling group. At his age, to be as fit as he is, it's wonderful for the others to learn from."
England collapsed themselves at Lord's in the second Test, and Root was thrilled with the response his side showed in Yorkshire.
"It was a fantastic, clinical performance, the two big lads were exceptional, built pressure early on," he said.
"Three maidens this morning, and we felt we were in a good place to take wickets. When chances came, we took them.
"Look at the talent in the dressing room – we know we're capable, we just have to be more consistent."
It was a sentiment echoed by Anderson, who singled out Robinson (5-65 in the second innings) for praise.
"It feels good. [It was a ] huge disappointment at Lord's, we wanted to come back and put in a better performance and we did that," Anderson told Sky Sports.
"As a group we bowled brilliantly. There were nerves, because of the quality of India's batting line-up, we thought if they get stuck in and get a big partnership it could be a long day.
"Ollie Robinson today was unbelievable, he's been brilliant since he came into the side. But I think it's that collective effort, we were relentless with that yesterday and carried it on today."
India captain Virat Kohli, meanwhile, was left to reflect on a match gone badly wrong.
"Quite bizarre, it can happen in this country, batting collapses," he said.
"We thought the pitch was good to bat on, coming on nicely. But the discipline forced mistakes and the pressure was relentless. It's difficult to cope when you're not scoring runs.
"As a batting group we need to stay close and confident, even after 36 all-out [in Australia] we came back."
The magnificent Anderson produced one of the great spells before lunch in Chennai on day five of the first match of the series, putting the tourists well on their way to a 227-run victory.
Anderson generated huge reverse swing, taking three wickets for six runs in five overs during the morning session, and India could not contend with his skillset as the 38-year-old finished with brilliant figures of 3-17.
Impressive spinner Jack Leach took 4-76 as India were bowled out for 192 after lunch, half-centuries from captain Virat Kohli (72) and Shubman Gill (50) proving to be in vain.
It was a dream 100th Test for Root, who made a sublime double century and was named man of the match after matching Michael Vaughan's record of 26 victories as skipper for England in the longest format.
Root paid tribute to the ageless Anderson after his final-day heroics.
The in-form batsman said: "He is the GOAT of English cricket. He finds ways of constantly challenging himself and getting better.
"At 38 to still be getting better and be as fit as ever is fantastic - he is a great role model. His skill level is as good as anyone you'll ever see."
Root added: "He just keeps getting better and better, like a fine wine. His skill levels are as high as ever and his fitness levels.
"He sets a great example for all the younger bowlers in our squad and I can see him playing on for many more years, which is very exciting."
On England's overall performance, Root said: "It's been a fantastic five days with the way we came back and executed the game plan, it was really, really pleasing and we'll enjoy this before getting ready to come back again."
Brook backed up his century in the first Test with another in Wellington, before Root reached three figures in the second innings.
The pair are ranked first and second in the ICC Test match rankings – Root on top and his fellow Yorkshireman behind him – but the former England captain insists Brook is the better player.
Root told BBC Sport: "To be able to construct an innings like that off the back of another exceptional hundred the week before, with such an all-round game - he can absorb pressure, he can apply it.
"He can whack you over your head for six. He can scoop you over his head for six. He can smack spin. He can smack seam.
"Brooky is by far and away the best player in the world at the minute."
Brook's Test average of 61.62 is the third-highest of any batter to have played at least 20 Test innings, while 2,280 runs from 23 Tests is the most of any England player in history after that number of games.
Only four men have made more Test centuries than Root's 36, though, and Brook was full of praise for his senior partner.
He said: "I'm trying to catch him but he [Root] is too good, isn’t he? He’s obviously one of the best players, if not the best player, to have ever played the game.
"It's mega to play with him and he’s awesome to watch."
Root is the leading scorer (1,470) in Test cricket in 2024, having struck a career-joint-best six 100s in the year.
The final Test of England's three-match series against the Black Caps begins in Hamilton on Saturday as the tourists aim for a whitewash victory.
Root declared with the tourists 185-5 after batting on during a morning session on day five that included short delays due to rain in Barbados.
The Windies were never likely to chase down a target of 282 to win, but Brathwaite led by example with 56 not out after making 160 in a marathon first innings, to frustrate England with support from Joshua Da Silva (30no) and Jermaine Blackwood (27).
West Indies batted out 65 overs and were 135-5 when Root shook hands with Brathwaite, setting up a series decider in Grenada.
Opener Brathwaite set a record for the most balls faced by a West Indies batsman in a single Test – 673 – on the back of his incredible first-innings vigil.
Jack Leach took 3-26 and debutant Saqib Mahmood 2-21 on the last day as England were unable to force a victory, having also failed to bowl West Indies out on day five in Antigua.
Root had no regrets about not ending England's second innings earlier in the day.
He said in the post-match presentation: "It's always a tricky one. With how small this ground is and how strong the winds are, a couple of overs of someone coming off – you don't want to make it too close."
Root praised spinner Leach, who racked up 94.5 overs in the match on a flat pitch.
"It's really pleasing to see Jack play the way he has. You can see how much he enjoys it out there," Root said. "He's bowling with great control and looks threatening. It's great to see him find his feet and look comfortable at this level."
Marathon man Brathwaite was delighted by the fight his side showed under pressure.
He said: "I thought it was a tremendous effort. England bowled extremely well. It was good as a team we could fight and put 400 back. Fighting at the end, that is the attitude we want."
Brathwaite wants to see a surface in Grenada that will give the Windies paceman more of an opportunity to do damage.
He added: "We put in a very big effort, but we want a result in the last Test. We have to see what they produce [the pitch for the final Test] but let's hope it has something for the pacers."
Joe Root was replaced as skipper by Ben Stokes ahead of the series but maintained a key role against the Black Caps and was named England's player of the series.
He was far from alone in standing out, though, and far from alone in enjoying himself, as England became the first Test team to chase down 250 three times in a single series.
The third win was completed at a canter on day five at Headingley, with Root joined in the middle by the destructive Jonny Bairstow, illustrating a complete shift from what had gone before.
Prior to Root – the world's top-ranked batsman – giving up the captaincy, England had just one win in 17 Tests.
"One of the great things about this series was guys keep getting more and more confident, more and more at ease with how they want to go about things," Root said.
"It's such fun to play in and be part of. For Ben to start as he has under Brendon [McCullum, the new head coach] as well, we're all loving playing it; it's great.
"It's important that we enjoy this series win. It has been a rocky road for the Test team for a little while now, and if you can't enjoy a series like this, then what do you play for?"
Those words would be music to Stokes' ears, with results secondary to the enjoyment of his team, he says.
"For me, when I took over this job, it was more than results; it was about changing the mindset of the lads towards Test cricket, about having fun and enjoying the fact you're out there representing your country, and the results will look after themselves," Stokes said.
"But to say that we have done it so quickly is just unbelievable.
"I can only do so much; I've got to show a huge amount of credit to Brendon and the way that he's came in and influenced this group as well, the backroom staff and everyone else who's played a part in the series as well – it's been absolutely phenomenal."
Stokes described this series as "a pretty special start" and picked out the third Test as his highlight, impressed England did not "come back into our shell" when they were 55-6 in their first innings.
Those bad habits have been forgotten, it seems, and Root was not interested in reflecting on what had gone wrong during his tenure.
"I think we should leave what's happened where it is," he said. "I think we should concentrate on looking at what this team has done over the last few weeks.
"What Ben's done has been brilliant in these three games, and I'm sure he'll have a plan for the next series and this game against India as well.
"As I say, I'm absolutely loving being a part of it all at the moment. Long may it continue."
That India match concludes a series that was started last year with Root at the helm, but Stokes intends to attack it in the same manner England did world Test champions New Zealand.
The new skipper added: "It's obviously completely different opposition and we've still got a series to try to draw, but we'll be thinking about us, and trust me when I say this: we'll be coming out with exactly the same mindset."
