Sinner's first ATP Tour final had come at this event in 2020, where he opened his account by beating Vasek Pospisil in three sets – a sterner test than he was given by Monfils.
The Italian had since tasted victory twice more, while Monfils' previous final had been in February 2020, although his success this week extended a run of seasons with final appearances to 17 in a row.
The most recent three of the Frenchman's prior 31 had ended in glory, but he was no match for defending champion Sinner.
The 20-year-old had already passed up a break point before consecutive superb forehand winners brought the breakthrough, the second clipping the net on its way through.
A marathon rally saved the second of two break points Sinner faced in the seventh game, yet that was as close as he came to being stopped.
After Sinner served out the opener, Monfils paid the price for losing his focus at the start of the second, adamant he had landed the first point of the set inside the line as a narrow call went against him.
Monfils' wild backhand then went long to give Sinner the lead in the second, one he never relinquished as he coasted through to serve out the match to love.
"I'm happy to be one more time the winner here in Sofia," Sinner said. "I think it's a very, very nice tournament.
"For sure it was the best match [he played in the tournament], because I had to. It's never easy to play against him. We've had very tough matches, I think everyone knows that.
"Today, in the [key] moments, I was a little bit luckier than him, so that helped a lot."
Sinner won his fourth title of what has been an outstanding season for the Italian in Antwerp last weekend and is only 110 points adrift of Hubert Hurkacz in the battle for the final place in the season-ending event in Turin.
Murray did the 20-year-old a favour by knocking Hurkacz out of the ATP 500 tournament in the Austrian capital on Monday and seventh seed Sinner eased to a 6-4 6-2 win over American Opelka two days later.
Sinner, who will overtake Pole Hurkacz if he reaches the semi-finals this week, won 93 per cent of points behind his first serve and did not face a break point in a resounding win and will play Dennis Novak in the second round.
Murray beat Alcaraz at Indian Wells this month, but the 18-year-old rising star from Spain turned the tables on the three-time grand slam champion with a 6-3 6-4 second-round win in Vienna.
Former world number one Murray struggled with his serve and was broken five times as he made an early exit.
Third seed Matteo Berrettini beat Nikoloz Basilashvili 6-7 (5-7) 6-2 6-3 to reach the third round, while Diego Schwartzman, Gael Monfils and Lorenzo Sonego moved into the second round.
In the St Petersburg Open, defending champion Andrey Rublev beat Ilya Ivashka 6-4 6-4 to reach the quarter-finals along with Denis Shapovalov, a 2-6 6-3 6-0 winner against Pablo Andujar.
Jan-Lennard Struff also moved into the last eight, with Karen Khachanov and Marin Cilic securing first-round wins.
The top seed won just two points across the first three games but took a first set that featured four breaks and lasted over an hour.
Duckworth broke in the first game of the second but eventually ceded that advantage and the frustrated Australian could not prevent Sinner from breaking again to claim a 7-6 (7-4) 6-4 win to set up a last-four clash with Filip Krajinovic, who prevailed in three sets against Kamil Majchrzak.
Gael Monfils will feature in the other semi-final after brushing aside Gianluca Mager 6-2 6-2, booking a meeting with American Marcos Giron.
It is a second semi-final in as many weeks for Monfils, who also reached the last four in Metz.
Giron will contest his first Tour-level semi-final following a 6-4 6-2 defeat of John Millman.
"Nothing is easy," Monfils said. "We have been putting in work for nearly four days here. Working on my serve and my forehand and it worked perfectly today. I will be ready for Marcos. He has been playing very well, so I think it will be a tough semi-final."
The top seed is targeting his third ATP title of the season, having already triumphed in Melbourne and Washington.
And the Italian is yet to drop a set in the Bulgarian capital as he boosted his bid to reach next month’s ATP Finals with a 6-3 7-5 success.
Breaks were exchanged in the opening set before Sinner edged his nose in front, while he recovered from 5-2 down in the second by reeling off five successive games to prevail.
"It was a very tough match against [Krajinovic]," Sinner said. "We practise at times together, so we know each other's games a little.
"There was a chance to go to a tie-break, which I luckily avoided. I am happy to be in the final here again."
His opponent in the final will be second seed Gael Monfils, who also advanced in straight sets after defeating Marcos Giron 7-5 6-0.
Though 4-2 down in the opening set, the world number 20 won 11 of the next 12 games against the American, who was appearing in his first tour-level semi-final having defeated Alex de Minaur along the way.
The Frenchman's progression means he has now reached at least one ATP final in each of the last 17 years; a tally only eclipsed by Rafael Nadal (18).
"I am very happy as I was very solid - it was a very physical match with lots of long rallies," said Monfils, who also reached the semi-finals in Metz last week.
"I felt in his rhythm he was crushing the ball, so I wanted to change the speed a little and come to the net a little and step in. I think that was key for me."
Monfils is the ATP world number 21, while Ukrainian Svitolina is ranked 27th in the WTA rankings.
On Sunday, the pair, who married in 2021, announced the news on their official Twitter accounts.
"With a heart full of love and happiness, we are delighted to announce that we are expecting a baby girl in October," a post on each of their accounts read.
Svitolina has been outspoken about her opinion on Russian and Belarusian athletes participating on the Tour, in the wake of Russia's invasion of her homeland.
In April, the 27-year-old called for Russian and Belarusian players to be banned from all international tennis events unless they denounce the invasion in Ukraine.
She decided to take a break from tennis in March, citing a back problem.
The couple, who married in July 2021, have announced the arrival of daughter Skai Monfils.
Former world number three Svitolina and two-time grand slam semi-finalist Monfils announced last May they were expecting a baby girl.
Svitolina posted on Instagram on Saturday: "Ladies and gentlemen, please give a warm welcome to Skai Monfils.
"Can't thank enough my husband to live [through] this unforgettable moment with me."
Monfils wrote on social media: "I had the most amazing night of my life, which ended with the most beautiful gift around 6.00am.
"Elina was strong and brave. I can't [thank] enough my wife and God for this special moment. Welcome to the world my little princess Skai."
Number two seed Thiem retired due to a wrist injury when 5-2 up in the opening set against Adrian Mannarino in the Mallorca Championships on Tuesday.
An MRI conducted at a Mallorca hospital was inconclusive, so the US Open champion will now undergo further tests.
In an update on Wednesday, Thiem wrote on social media: "Yesterday during the match I had a problem with my wrist.
"I went immediately to do an MRI at the hospital in Palma de Mallorca. The results weren't that clear and I have decided to go to Barcelona to check with a specialist.
"I hope I can get the results and a clear diagnosis in the next days."
The ATP 250 event continued without him in Mallorca, as Roberto Bautista Agut moved into the quarter-finals with a 6-3 7-5 triumph over Italian Stefano Travaglia.
More Spanish success on home soil arrived as Pablo Carreno Busta won 6-4 6-4 against Jiri Vesely.
In the doubles, Novak Djokovic and Carlos Gomez-Herrera saw off top seeds Marcel Granollers and Horacio Zeballos to reach the last four in that competition.
Djokovic appears to be enjoying himself as he continues to prepare for Wimbledon, saying: "This was a huge win for us, beating one of the best doubles players in the world after losing the first set.
"We are having a lot of fun on the court. I thought we played well even though we lost the first set."
There was a major upset at the Viking International in Eastbourne, meanwhile, with number one seed Gael Monfils suffering a shock last-16 defeat to Australia's Max Purcell – ranked number 283 in the world.
Purcell, who is only in the tournament as a lucky loser, claimed a huge 6-4 5-7 6-4 win in a battle lasting over two hours.
Monfils fired down 16 aces and fought back after being within two points of defeat in the second set but was ultimately beaten and has not made an ATP quarter-final since February 2020.
"It feels unbelievable," said Purcell. "I thought I'd come out and have a go.
"I've struggled to get into any singles events over the past nine months and primarily played doubles, so to get on a run here, on my favourite surface, is great."
Purcell will take on Andreas Seppi – a comfortable winner over Emil Ruusuvuori – in the last eight.
While Monfils crashed out, there was less drama for the second and third seeds.
Alex de Minaur won 6-3 6-4 against home hope Liam Broady, while Lorenzo Sonego was a 6-4 6-2 victor in his contest with John Millman.
World number 84 Ruusuvuori saved a set point in the first-set tie-break on his way to a 7-6 (8-6) 6-4 victory and will now face Ilya Ivashka for a place in the semi-finals.
There was nearly another upset as home hopeful Alexander Bublik, seeded second, had to recover from a set down to overcome Miomir Kecmanovic 2-6 6-3 7-5.
Carlos Taberner awaits Bublik in the next round after the Spaniard beat Egor Gerasimov 5-7 7-6(5) 7-5.
Kwon Soon-woo and James Duckworth saw off Dusan Lajovic and Filip Krajinovic respectively in straight sets to remain a course for glory.
At the Moselle Open in Metz, meanwhile, Gael Monfils took out Philipp Kohlschreiber 7-6(2) 6-4 to reach the quarter-finals on home soil.
Tournament favourite Hubert Hurkacz made light work of 2016 champion Lucas Pouille with a 6-2 6-3 victory to set up a meeting with Andy Murray in the quarter-finals.
Elsewhere on Thursday, German qualifier Peter Gojowczyk proved too strong for Karen Khachanov and will now face Marcos Giron, who upset fourth seed Alex de Minaur.
Medvedev – the 2019 US Open finalist – was playing his first match since a five-set defeat to Stan Wawrinka at the Australian Open but was unable to find the same level he produced in that thriller.
Instead, Medvedev converted just one of his six break point opportunities as Pospisil claimed a 6-4 6-3 victory on Wednesday.
Sixth seed Roberto Bautista Agut was also on the receiving end of a surprise result, compatriot Pablo Carreno Busta winning 6-4 2-6 7-6 (7-4) in the second round.
With Medvedev and Bautista Agut out of the draw, Felix Auger-Aliassime can afford to harbour greater hope of winning his first ATP singles title after he saw off Grigor Dimitrov 6-4 6-2.
Though Auger-Aliassime is a place higher in the rankings than Dimitrov at 21st in the world, some may have expected the Bulgarian's greater experience to win out.
However, it was the 19-year-old Canadian who controlled matters, enjoying great success against the second serve of Dimitrov, who committed 27 unforced errors as Auger-Aliassime progressed to the quarter-finals.
Daniel Evans will join him in the last eight after the Briton defied the odds to come from a set down and defeat Karen Khachanov 4-6 6-3 6-4.
Gael Monfils, David Goffin and Gilles Simon all sealed first-round wins, though the latter pair did so in unconvincing fashion.
While third seed Monfils eased past Joao Sousa 6-3 6-2, fourth seed Goffin had to come from behind to knock out Robin Haase 3-6 7-6 (7-5) 6-4 and Simon survived a second-set blip in a 7-6 (7-3) 3-6 6-3 win over Mikhail Kukushkin.
Jannik Sinner is also through to round two, the 18-year-old given a walkover after Radu Albot withdrew with a shoulder injury.
After top seed Daniil Medvedev departed the competition a day earlier, Tsitsipas and David Goffin - seeded second and fourth respectively - each suffered upsets to blow the bottom half of the draw wide open.
Slovenia's Aljaz Bedene recorded the biggest win of his career as he cut down Tsitsipas in straight sets, his second triumph over a top-10 player clinched by a 7-5 6-4 scoreline.
"I tried to stay focused, and I delivered," Bedene said shortly after a stunning winner down the line saw him successfully serve out for the match.
"I actually served really well when it was important and kept my cool really, because I was feeling that he [Tsitsipas] was pressuring sometimes.
"I had to stay focused and cool and play my shots, and I delivered. I'm happy with that."
The world number 52 saved five break points in the opener before claiming his first opportunity, and he conceded: "I think he [Tsitsipas] was probably the better player for most of the first set.
"He was serving really well, and I didn't play my game - especially on his service points. I stayed tough, which I had to do, otherwise it would go the other way. I guess the break gave me a boost of confidence."
Bedene will play Felix Auger-Aliassime in the quarter-finals, with the winner potentially meeting NextGen champion Sinner - matched with Pablo Carreno Busta - in the last four.
Sinner, still just 18, came up with 27 winners to see off 10th-ranked Goffin 7-6 (9-7) 7-5.
"I feel great, especially now in this moment," the Italian said, according to quotes on the ATP Tour website. "It was not easy playing against him. He is very solid, so you sometimes go for a winner or change the rhythm.
"It feels like a normal victory, but sometimes you play better than others. It's all about improving and testing where you are."
Reigning champion Gael Monfils prolonged his title defence, beating countryman Gilles Simon 6-4 6-1.
Seventh seed Andrey Rublev also advanced, while Vasek Pospisil could not build on his shock defeat of Medvedev, going down 6-4 7-6 (9-7) to Filip Krajinovic.
The Pole beat Roger Federer on the way to reaching the semi-finals in 2021, while he was runner-up to new world number one Jannik Sinner in Halle last month.
However, when trailing by two sets to one against his French opponent, Hurkacz sustained a knee injury during the tie-break in the fourth.
Despite valiantly attempting to battle on, he aggravated the problem further soon after when diving for a return, and was subsequently unable to continue with Fils advancing to round three.
There, he will play Alex de Minaur after the ninth seed enjoyed a straight-sets victory over Jaume Munar.
Fourth seed Alexander Zverev also prevailed in three sets as he continued his rich vein of form. The Italian Open champion, who was also runner-up to Carlos Alcaraz at Roland-Garros, was a commanding 6-1 6-2 6-4 winner over Marcos Giron.
Next up for him is Cameron Norrie after the 2022 semi-finalist ousted compatriot Jack Draper in a battle of the Brits on No.1 Court.
Eastbourne champion Taylor Fritz overcame Arthur Rinderknech in four sets, as did 15th seed Holger Rune against Thiago Seyboth Wild.
Meanwhile, Grigor Dimitrov recovered from two sets down to beat Juncheng Shang in a decider. The 2014 semi-finalist will now play Gael Monfils, who completed a straight-sets victory over fellow veteran Stan Wawrinka.
Although, there was no joy for 11th seed Stefanos Tsitsipas. The two-time major finalist was beaten in four sets by world number 87 Emil Ruusuvuori.
The German world number three was bounced out at Melbourne Park by Denis Shapovalov in straight sets, but he navigated his way safely past Mackenzie McDonald to reach the quarter-finals in Montpellier.
A largely dominant Zverev won 6-2 7-6 (7-5) to set up a clash with Adrian Mannarino in the last eight, and the 24-year-old highlighted how important he felt it was to quickly get back in the swing of things after disappointment in Melbourne.
"I'm going to give myself the best chance to do well in this tournament. But obviously, I'm happy to be back here," he said of the tournament he won in 2017.
"This is a place I really enjoy coming back [to]. After the Australian Open that I had, I wanted to play tournaments before my scheduled tournaments in Acapulco, Indian Wells and Miami. I said Montpellier is the perfect fit for that, and I'm happy to be here."
The same cannot be said for Gael Monfils, who suffered a crushing 6-1 6-2 defeat to Mikael Ymer on home soil.
The Swede was practically flawless throughout, winning a whopping 83 per cent of points on first serve and 89 per cent on second, with Monfils never even sniffing out a single opportunity to break back.
Richard Gasquet is a force to be reckoned with in Montpellier and reached another quarter-final, seeing off South Korean's Kwon Soon-woo 7-5 6-4.
Frenchman Gasquet reached six consecutive finals in Montpellier while in his prime years, being crowned champion in 2013, 2015 and 2016, and finishing runner-up in 2014, 2017 and 2018. Now 35, his last quarter-final appearance in an ATP main tour event came in August at Winston-Salem.
Gasquet's fellow Frenchman Mannarino sank the hopes of defending champion David Goffin, scoring a sparkling 6-4 6-2 victory.
Fourth seed Nikoloz Basilashvili, who lost twice to Andy Murray in Australia last month, this time fell to world number 152 Damir Dzumhur of Bosnia-Herzegovina, sliding to a 6-1 5-7 6-2 defeat. Basilashvili, the world number 21, now has a record of played five, lost five, for the 2022 season.
At the Tata Open Maharashtra in Pune, there was a surprise defeat for Russian top seed Aslan Karatsev. Karatsev won a title in Sydney last month before losing to Mannarino in the third round of the Australian Open, and in his first match since that disappointment he slumped 6-2 7-6 (7-3) to Swedish qualifier Elias Ymer.
Three seeds joined Ymer in the quarter-finals, with Jiri Vesely, Emil Ruusuvuori and Stefano Travaglia all making smooth progress without dropping a set. Vesely faces Ruusuvuori on Friday, while Travaglia awaits Ymer.
Zverev, who last in last year's final, saw match points slip from his grasp in the second set as Dimitrov levelled the match, however, he eventually prevailed on Thursday.
World number one Novak Djokovic benefited from a walkover, while Daniil Medvedev also moved through to the quarter-finals of the ATP Masters 1000 event.
Zverev outlasts Dimitrov
Olympic Games gold medallist Zverev had to dig deep for his 7-6 (7-4) 6-7 (3-7) 6-3 win over former world number three Dimitrov.
It was Zverev's seventh win on the bounce, but he was well aware of just how deep he had to dig to see off a resilient Dimitrov.
"Incredible match," Zverev, who won a fifth tour title of the year last week at the Vienna Open, said afterwards. "Grigor is playing very high-level tennis and I had to leave everything out there.
"I think it was a very high-level match and it could have gone both ways, especially the third set. I am happy with how things went in the end."
Djokovic moves through without playing
There was no such ordeal for top seed Djokovic, however.
Djokovic – a 20-time grand slam champion – received a walkover after Gael Monfils was advised not to continue with the tournament due to an adductor issue sustained in his previous win over Adrian Mannarino.
Taylor Fritz awaits five-time Paris champion Djokovic after the American saw off 10th seed Norrie 6-3 7-6 (7-3), while Casper Ruud will tussle with Zverev.
Ruud's emphatic 6-2 6-1 demolition of Marcos Giron clinched a historic achievement for the 22-year-old – who became the first Norwegian to qualify for the ATP Finals at the end of a breakthrough year in which no player can better his five tour-level titles.
Hubert Hurkacz did his chances of joining Ruud in Turin no harm as the Polish seventh seed – who holds the last automatic qualifying berth – fought back to defeat Dominik Koepfer 4-6 7-5 6-2 and set up a meeting with James Duckworth, who beat Alexei Popyrin 7-6 (8-6) 6-4.
Second seed and US Open champion Medvedev was another who had to come from behind before ultimately seeing off Sebastian Korda 4-6 6-1 6-3.
He will go up against Hugo Gaston, who became the first French qualifier to reach the Paris Masters quarter-finals since 1990 with his win over Carlos Alcaraz.
Olympic Gamed gold medallist and third seed Zverev fell behind a break in both sets but battled past Murray in straight sets at the ATP Masters 1000 tournament on Tuesday.
Tsitsipas – the second seed – rallied from a set down to vanquish Fabio Fognini in the desert, while fifth seed Berrettini was a third-round casualty.
ZVEREV CLAIMS COVETED SCALP OF MURRAY
Having already defeated Novak Djokovic, Rafael Nadal and Roger Federer in his career, all that was missing for Zverev was a win over Murray to complete the 'Big Four' sweep.
Zverev added Murray to his list of scalps with a 6-4 7-6 (7-4) victory to reach the Indian Wells fourth round for the first time since 2016.
The German star has now won 19 of his last 20 matches since Wimbledon.
"He's the only one of the Big Four that I hadn't beaten yet, so I'm happy that I've done it today," said Zverev, who will clash with 14th seed Gael Monfils in the next round. "Obviously it was a fantastic match.
"I thought Andy played extremely well, maybe as well as he's played since the surgery. I hope he continues playing the same way, because tennis did miss him for a long time and I think it's good to have him back."
TSITSIPAS FIGHTS BACK
It was far from easy for Greek star Tsitsipas, who prevailed 2-6 6-3 6-4 against 25th seed Fabio Fognini.
Tsitsipas added to his ATP Tour-leading haul of match wins this season, which now stands at 53 after Fognini had been looking to score his first victory over the French Open runner-up.
Alex de Minaur – the 22nd seed – awaits after he took down 13th seed Cristian Garin 6-4 6-2 for his first trip to the Indian Wells last 16.
BERRETTINI BUNDLED OUT
Wimbledon finalist Berrettini was no match for Taylor Fritz, who surprisingly topped the Italian 6-4 6-3.
Berrettini entered the contest as the only player on the ATP Tour this season to register double-digit wins on three surfaces – 15-4 (hard), 13-4 (clay) and 11-1 (grass).
"We're coming to the end of the year, I could really use a big result," said Fritz after claiming his first top-10 win of the year. "This is just what I needed, playing one of my favourite tournaments close to home."
