The 18-year-old struggled for rhythm in the opening set but showed composure to respond by breaking Munar early in the second set and dominating from there.
Alcaraz took the victory in two hours and 15 minutes, saving 10 of 13 break points across the match, while dropping only three games in the second and third sets.
Spaniard Pedro Martinez set up a second-round clash with third seed Diego Schwartzman after a 6-3 6-4 win over Shang Juncheng.
Pablo Andujar Alba also progressed 7-6 (7-2) 7-5 over Pablo Cuevas, while Serbian qualifier Meomir Kecmanovic accounted for local wildcard Felipe Meligeni Rodrigues Alves 6-3 6-0.
At the Delray Beach Open in Florida, Steve Johnson beat South African veteran Kevin Anderson in just over two hours, triumphing 6-2 6-7 (5-7) 6-2.
Oscar Otte got past Yoshihito Nishioka 7-6 (10-8) 6-3 to set up a second-round date with top seed Cameron Norrie, while there were only two breaks across the match as seventh seed Maxime Bressy was ousted by John Millman 6-7 (2-7) 7-6 (7-2) 7-6 (7-3).
Italian veteran Andreas Seppi beat German Peter Gojowczyk 7-5 6-4, while young American Brandon Nakashima comfortably handled Denis Kudla 6-1 6-1.
Thiem earned his first tour-level win in 14 months by downing Emil Ruusuvuori on his Bastad debut, before reaching his first quarter-final of the season by overcoming Roberto Bautista Agut on Thursday.
The Austrian battled to a 7-6 (7-5) 3-6 6-4 victory over the fourth-seeded Bautista Agut, picking up his first top-20 win since beating Novak Djokovic at the ATP Finals in 2020.
Sebastian Baez, who defeated Alejandro Davidovich Fokina in straight sets, awaits in the next round and former world number three Thiem is ready for the task.
"The win against Emil gave me confidence that I can compete against the best players in the world and today against Roberto," said Thiem, the 2020 US Open champion.
"He is always such a tough opponent. I was expecting and hoping it would be a close match. Then when I won the first set in the tie-break I said 'Hey, I can win today.'
"I had the belief until the end of the match and the third set was really good. It is a process. I beat Ruusuvuori and today I beat Bautista Agut.
"Two really top opponents, so if I can beat them I am definitely back. I am into the quarters of a really strong tournament, so I am happy. It is a process, but I am going the right way."
Second seed Andrey Rublev was made to work for his 7-5 4-6 7-6 (7-2) victory over Federico Coria and will meet Laslo Djere, who defeated Marc-Andrea Huesler in straight sets to reach the quarter-finals.
Meanwhile, Auger-Aliassime resumed his match with Jason Kubler in Newport that was suspended due to bad light on Wednesday with a slight advantage but fell to a 4-6 6-3 7-6 (7-4) second-round loss.
That marked a first top-10 victory for Kubler, continuing his fine 2022 season that saw him achieve his best major result at Wimbledon, where he reached the fourth round at the All England Club.
Fourth seed Maxime Cressy is yet to drop his serve at the tournament but needed to survive a first-set scare against fellow American Steve Johnson to triumph 6-7 (3-7) 6-4 6-4.
Second favourite John Isner will meet Cressy in the semi-finals after battling past fifth seed Benjamin Bonzi 6-7 (2-7) 7-6 (8-6) 7-6 (7-5).
The Swiss great opted against playing a lead-up tournament, but showed no signs of rustiness in a 6-3 6-2 6-2 victory over Johnson at Rod Laver Arena.
Federer, a 20-time grand slam champion, was in control throughout, with not even a first-set rain delay able to halt a strong performance.
A six-time winner in Melbourne, Federer had won his previous two meetings with Johnson in straight sets, and he made it three in a row.
Federer took early control against Johnson, racing out to a 3-0 lead before rain delayed play after the fifth game.
But, the brief pause did little to slow Federer, especially under the Rod Laver Arena roof.
He took the first set and grabbed a 4-0 lead in the second, another break to begin the third seeing him on his way to a convincing victory.
Federer will face either Filip Krajinovic or qualifier Quentin Halys in the second round.
Despite not playing a lead-up tournament, the Swiss great looked in fine form in a 6-3 6-2 6-2 victory over Steve Johnson on Rod Laver Arena.
Federer, a 20-time grand slam champion, said the early rounds would be key after opting against competitive matches ahead of the year's first major.
"I just haven't played proper matches in many, many weeks, and a lot of guys, probably 95 per cent of the guys, are coming here with matches. I'm not one of those guys," he told a news conference.
"Now I have one. Best of five, too, which is even better. I think for me really the first three rounds are key to get going, to get used to the pressure, stay calm, when to save break point or 30-30 points or whatever it may be or just to stay calm if you're down a set and a break or whatever it might be.
"This is sort of the unknown that can be a little bit scary at times. But today there was none of that because I broke early each set and was able to get on a roll, play freely after that. And also felt I had margin.
"Anything I was doing I felt like I had the game under control. That might not be the case in the next round, so I just think I have to be careful.
"Round by round, point for point mentality. I know other guys that are playing extremely well right now so I think it's just important to stay very calm about things right now."
Federer produced a polished display against Johnson, hitting 34 winners and 20 unforced errors.
And, asked about the court speed, Federer said it was similar to last year at Melbourne Park.
"I think balls play fast when they are new, a ball change for a couple of games, depending on who you play, how long the rallies are right then," he said.
"But I'd say two to four games it can play faster. But the balls fluff up extremely quickly here when you do get into long rallies. And I feel night sessions or indoor or on a cool day like what we will see in the next week, actually play quite slow. It is what it is, you know.
"But I think it depends on how you play maybe also and how you manage your game and what kind of opponent you have, for all sort of playing styles, I guess."
The Swiss 20-time grand slam champion decided against playing a lead-up tournament ahead of the year's first major, where he faces Steve Johnson.
Despite the lack of competitive matches, it would still take a monumental upset to send Federer packing in the opening round.
We take a closer look at where the six-time champion in Melbourne is at ahead of the first round.
Form and results
Federer has been out of competitive action since the ATP Finals in November, when he suffered a semi-final loss to Stefanos Tsitsipas. The 38-year-old did edge Nick Kyrgios 7-6 (8-6) in an exhibition set at the 'Rally for Relief' event on Wednesday.
First up
In contrast, his first-round opponent, Johnson, is coming off a Challenger Tour title win in Bendigo. A former world number 21, the American almost dropped out of the top 100 last year, but already has seven wins at Challenger level in 2020. Federer holds a 2-0 head-to-head record over Johnson, but was pushed to two tie-break sets in their last meeting at Indian Wells in 2017. Federer's last first-round loss at a major was at the 2003 French Open, while he is 20-0 in the Australian Open first round.
Draw
Federer is in the bottom half of the draw and will face either Filip Krajinovic or qualifier Quentin Halys if he gets past Johnson. If results go by ranking, Hubert Hurkacz will await in the third round.
What he said
"I'm excited to play Steve. He's a good guy. I think with his old-school playing – big forehand, slice backhand, good serve – I think it's going to be a nice match for me, as well."
Fresh off his maiden ATP 1000 title at Indian Wells and fourth-round run in Miami, the American world number 13 needed just 62 minutes to win in straight sets, in his first clay court match for the season.
Fritz's serve was not particularly secure, though, with the unseeded Tabilo converting on only one of six break point attempts. Fritz was more efficient and assertive when opportunities presented on the other hand, taking two of three break point chances on Tabilo's serve while winning 12 out of 19 points returning on his second serve.
Despite the Chilean world number 100 fighting back to 4-4 from 3-0 down in the second set, Fritz steadied to close out the match.
"I felt like tonight was going to be a really tough match, given he's done really well on clay," Fritz said post-match. "It was my first match [on clay]. I'm pretty happy with how I played. I can't complain, I got out of that pretty solid."
Fritz will now face another Chilean, fifth seed and defending champion Cristian Garin, who came back from a set down to defeat Jordan Thompson 3-6 6-3 6-3.
Entering the Houston event on a five-match losing streak, Garin has come from behind in back-to-back matches to win.
Sixth seed Frances Tiafoe also went the distance on Thursday, earning a 5-7 7-5 7-6 (7-4) win over Uruguayan Pablo Cuevas.
With neither garnering break point chances in the third set, Tiafoe finally created breathing room by winning four straight points at 2-2 in the deciding tie-break, before closing out the match.
It sets up an all-American quarter-final with John Isner, who defeated Steve Johnson 6-7 (1-7) 7-6 (7-4) 6-3 in Thursday's final game.
Kubler and Verdasco endured a 71-minute first-set tussle, which the Australian edged 9-7 in a tie-break before dominating the second frame, dishing out a bagel in 29 minutes to win 7-6 (9-7) 6-0.
World number 71 Kubler progressed to the second round where he will take on Colombia's Daniel Elahi Galan who defeated China's Zhang Zhizhen 6-2 6-3.
Johnson set up a second-round clash with American compatriot and top seed Frances Tiafoe after getting the better of Facundo Bagnis 7-6 (7-5) 1-6 6-1 in one hour and 55 minutes.
USA's Kudla also progressed with a hard-fought 6-4 4-6 6-4 victory over Australia's Aleksandar Vukic in just under two hours.
Dutchman Gijs Brouwer won 6-4 6-4 over Aleksandar Kovacevic, setting up a meeting with big-serving fourth seed John Isner.
Raonic advanced to the Australian Open quarters before falling to eventual champion Novak Djokovic last month, however, the Canadian was stunned in New York on Wednesday.
Kwon weathered 33 aces from Raonic as the South Korean prevailed after two hours, 25 minutes at the ATP 250 tournament midweek.
"I'm really happy to win a tough match against Raonic," Kwon during his on-court interview. "He played well. It was a great match."
Next up for Kwon is either eighth seed Kyle Edmund or Dominik Koepfer.
While Raonic bowed out, fellow seeds Ugo Humbert and Miomir Kecmanovic progressed from the last 16.
French fourth seed Humbert eased past Marcos Giron 6-1 6-0, sixth-seeded Serb Kecmanovic defeated Paolo Lorenzi 6-3 6-3, while Andreas Seppi saw off Steve Johnson 7-6 (7-4) 6-3.
At the Argentina Open, fourth seed Coric was sent packing by Thiago Monteiro – who won 6-4 7-6 (8-6) in Buenos Aires.
Despite letting a match point slip at 6-5 in the second-set tie-break, Monteiro moved through to his first ATP Tour-level quarter-final since 2018.
Pedro Sousa awaits after the lucky loser accounted for Jozef Kovalik 7-6 (7-3) 7-6 (7-5) on the clay courts.
Dusan Lajovic – the third seed – outlasted qualifier Pedro Martinez 7-6 (8-6) 7-6 (8-6) to set up a quarter-final with eighth seed Casper Ruud, who dismantled Roberto Carballes Baena 6-1 6-0.
Coming off an epic three-hour duel Wednesday in his first match since the French Open final, Nadal fell to Lloyd Harris 6-4 1-6 6-4 on Thursday at the Citi Open.
After a short-lived maiden appearance at the Washington, D.C. tournament, the Spaniard will look ahead to Toronto and Cincinnati as he continues his preparations for the U.S. Open.
Harris, meanwhile, heads to a quarter-final matchup against veteran Kei Nishikori after recording the biggest win of his career.
Facing Nadal for the first time, the 24-year-old South African slammed 16 aces Thursday and saved four of six break points.
The 20-time grand slam winner, who battled a foot injury to outlast Jack Sock in his first match, did not have enough to answer the world number 50 on Thursday.
In addition to the top-seeded Nadal's defeat, second seed Felix Auger-Aliassime also went down Thursday, falling 6-3 6-4 to American wild card Jenson Brooksby.
With their exits, the tournament is now without its top four seeds.
That leaves fifth seed Jannik Sinner as the top player remaining after he defeated Sebastian Korda 7-6 (7-3) 7-6 (7-3).
Nishikori advanced by downing seventh seed Cameron Norrie 3-6 6-3 6-3, while John Millman knocked out eighth seed Reilly Opelka 6-3 7-6 (7-4).
Among unseeded players, Denis Kudla defeated the red-hot Brandon Nakashima 3-6 7-6 (7-3) 6-4, while Steve Johnston eased past Ricardas Berankis 6-2 6-1 and Mackenzie McDonald beat Illya Ivashka 6-4 6-4.
In addition to Harris-Nishikori, Kudla will face McDonald in the other quarter-final in the top half of the draw, while Sinner meets Johnson and Millman takes on Brooksby in the bottom half.
The 19-year-old Californian Nakashima, ranked 115th in the world, prevailed 5-7 6-3 7-6 (7-4) over world number 22 Raonic in their first meeting.
The big-serving Canadian had 10 double faults to go with his 27 aces, while Nakashima did not commit one double fault and managed 13 aces.
It was the latest in a string of recent wins over established players for Nakashima, who stunned Sam Querrey and John Isner last week at Los Cabos before falling to Cameron Norrie in the final.
Nakashima next faces Australia's Jordan Thompson, who beat Peter Gojowczyk of Germany 7-6 (8-6) 6-4 to reach the quarter-finals.
In the other half of the draw, fifth seed Taylor Fritz rallied to defeat fellow American Steve Johnson 6-7 (5-7) 6-4 6-1 in a rematch of a quarter-final at Los Cabos, also won by Fritz.
Fritz will face countryman Reilly Opelka, who outlasted Bjorn Fratangelo 7-6 (7-3) 7-6 (7-4). The 6-foot-11 Opelka pounded 25 aces to just three double faults, winning 84 per cent of his service points.
By far the most experienced player remaining in the field after Harris' upset of Rafael Nadal on Thursday, Nishikori prevailed 6-3 7-5 to avenge a loss earlier this year in their only prior meeting.
Nishikori had lost his last six ATP quarter-final matches dating to a trip to the semis in Barcelona in April 2019.
The 31-year-old next faces Mackenzie McDonald, who downed countryman Denis Kudla 6-3 6-2 to reach his second career ATP semi-final. He has never played in a final.
In the other semi-final, fifth seed Jannik Sinner will meet wild card Jenson Brooksby.
Sinner had little trouble with American veteran Steve Johnson in a 6-4 6-2 win as the Italian won 93 per cent of his first-serve points and saved all three break points he faced.
Brooksby maintained the strong return game that as carried him all week in a 6-1 6-2 upset of 11th seed John Millman.
The Australian did not manage an ace against the 20-year-old American, who won 54 per cent of the points on Millman's serve and converted five of seven break points.
At 130 in the world, Brooksby is the lowest-seeded Citi Open semi-finalist since John Isner reached the 2007 final while ranked 416th.
Rain wreaked havoc in Rio, where only two quarter-final matches were completed at the ATP 500 tournament on Friday.
Thiem's back is against the wall, with the Australian Open runner-up and world number four down 7-6 (7-5) 2-1 against Mager – who is featuring in his first ATP Tour quarter-final.
Lucky loser Attila Balazs and qualifier Pedro Martinez were also playing when the rain returned in Brazil, with the latter leading 6-2 2-2.
Cristian Garin managed to win through to the semi-finals after the third seed claimed his seventh successive victory by rallying past Federico Coria 2-6 6-3 7-5.
Next up for Garin is fifth seed Borna Coric, who defeated Lorenzo Sonego 7-6 (7-5) 6-3 on the Brazilian clay.
At the Delray Beach Open, second seed Milos Raonic and fellow big server Reilly Opelka will meet in the semi-finals.
Raonic beat Steve Johnson 7-6 (8-6) 6-3 and 2019 New York Open champion Opelka took down Kwon Soon-woo 6-3 6-4 at the ATP 250 event.
"It's going to be tough. He gets a lot of free points on his serve and you've got to make him play," Raonic said.
"You can't just make him play though because he goes for it from the centre of the court. You've got to move in, you've got to find a way to be aggressive."
Elsewhere, Yoshihito Nishioka overcame teenager Brandon Nakashima 3-6 7-6 (7-5) 6-4 and sixth seed Ugo Humbert eased past 2018 winner Frances Tiafoe 6-1 6-2.
The 2021 Wimbledon semi-finalist needed 109 minutes to claim a 7-5 6-4 win over Munar, assuming control by breaking in the seventh game of the opener before doing likewise early in the second set.
Canadian Shapovalov – who is seeded fourth for the event – had not featured since a dramatic loss to Andrey Rublev at the US Open earlier this month, but produced a consistent display in Seoul, not committing a single double fault during the match.
His reward for the victory is a last-eight tie with Radu Albot, who held his nerve in two tie-breaks to record a 7-6 (7-3) 7-6 (7-3) win over Steve Johnson.
In Wednesday's other match, Japan's Taro Daniel beat Ecuador's Emilio Gomez in straight sets in the duo's round-of-32 tie.
Top seed Casper Ruud begins his campaign against Nicolas Jarry on Thursday.
Now unranked, Sock needed a wildcard into the ATP 250 tournament and suffered a 6-3 6-4 loss to fellow American Marcos Giron.
A former world number eight, Sock won just one singles match last year – at the Laver Cup.
Giron created 10 break points and converted four on his way to victory in 76 minutes.
Coming off a run to the Australian Open quarter-finals, Sandgren suffered a 6-7 (5-7) 6-3 7-6 (7-3) loss to Steve Johnson.
Sandgren squandered seven match points in a last-eight loss to Roger Federer in Melbourne and the fifth seed was edged by Johnson.
Sixth seed Miomir Kecmanovic and Italian veteran Andreas Seppi posted wins over Tommy Paul and Damir Dzumhur respectively.
At the Argentina Open, Casper Ruud was the only seed in action and eased past Pablo Andujar 6-2 6-3.
Pablo Cuevas, Federico Delbonis and Thiago Monteiro also moved into the second round.
The American had lost to Corentin Moutet on the clay in Italy but was back to form in Antwerp in only his second meeting with the Serbian.
Lajovic won their previous encounter at Indian Wells in 2017 but Tiafoe saved two set points in the second set to prevail 6-3 7-6 (8-6) and progress to the second round.
Tiafoe was not the only player to knock out a seed on the ATP Tour on Monday, as his compatriot Steve Johnson claimed victory over Marin Cilic at the Cologne Championships.
Johnson spurned four set points as the opener was sent to a tie-break. However, he prevailed in the shoot-out and then broke the ninth seed to love in the first game of the second.
Cilic never threatened to break back as the 2014 US Open champion slipped to a surprise defeat.
