World number two Daniil Medvedev came from a set down to power past ATP Finals debutant Hubert Hurkacz 6-7 (5), 6-3, 6-4 and get his title defence off to a winning start in Turin on Sunday.
Olympic champion Alexander Zverev was also awarded a win in his opening match after Italian Matteo Berretini pulled out injured early in the second set with the German leading 7-6, 1-0.
Russian Medvedev, who won the US Open title in September for his maiden Grand Slam crown, allowed his Polish opponent to take the early advantage in the season-ending event following a tight tiebreaker but responded in style.
The 25-year-old fired 15 aces to Hurkacz's 12 and converted two break points on a fast court to seal victory in the Red Group ahead of potentially tricky challenges against Germany's Alexander Zverev and Wimbledon runner-up Matteo Berrettini.
"It was a very tight match," said Medvedev, looking to become the first back-to-back ATP Finals winner since Serb Novak Djokovic claimed four between 2012-15, in his post-match interview. "One good shot and the point is over.
"I think a lot of matches are going to be like this because the courts are super fast. The only two games I had break points I managed to break and that was the key."
Medvedev showed glimpses of the form that helped him beat Djokovic, Rafa Nadal and Dominic Thiem en route to the title on London last year, as he went up 3-0 in the second set after an early break.
He raised his level in the third set to close out the match, hitting more than 30 winners and not facing a single breakpoint in a clash lasting a little more than two hours.
World number three Zverev was made to work very hard against Berrettini for more than an hour before the Italian pulled out injured.
Berrettini had struggled to hold serve, having to save two break points on his very first service game alone, but his forehand repeatedly got him out of trouble.
Another two dizzying forehand crosscourt winners earned him his first two break and set points at 6-5 but he could not convert either.
Zverev also squandered two set points before clinching the tie break on his third chance after an hour and 19 minutes.
But after a game into the second set Berrettini needed a medical break for on-court treatment to his abdominal muscles and lasted only one more point before pulling out in tears.
"I don't know what to say because it is the worst feeling you will ever have as a player," Zverev said. "Obviously, I feel like crying.
"I thought the first set was incredible high level tennis."
Muguruza seals semi-final berth, Pliskova beats Krejcikova at WTA Finals
Spain's Garbine Muguruza reached the semi-finals of the season-ending WTA Finals in Guadalajara, Mexico, after beating Anett Kontaveit, while Karolina Pliskova missed the cut despite her comeback victory over Barbora Krejcikova on Sunday.
Muguruza ended Kontaveit's 12-match winning streak with a 6-4 6-4 win over the Estonian, booking a spot in the knockout stage for the first time since her tournament debut in 2015.
Muguruza won a remarkable 80% of her first serve points and fired seven aces in Sunday's victory as she finished second in the "Teotihuacan Group" behind Kontaveit, with both posting a 2-1 record.
"Today was a match that was kind of everything or nothing," Muguruza told reporters.
"Starting the round-robin with a loss, it was never good. To finally get two wins and qualify was amazing."
Two-time Grand Slam champion Muguruza will face compatriot Paula Badosa in Tuesday's semi-final. The winner of the contest will be the first Spanish woman to reach the final since Arantxa Sanchez Vicario finished runner-up in 1993.
"I didn't know all of this history," Muguruza said. "1993, I was born that year.
"It's amazing that there are four players left and two of them are Spaniards. That just shows that Spain has a great level of tennis."
Earlier, Pliskova came out on top in an all-Czech showdown, storming back to stun compatriot Krejcikova 0-6, 6-4, 6-4.
Despite the win, Pliskova failed to reach the semi-finals as she finished third in the group, with Kontaveit and Muguruza advancing as the top two. Second-seeded Krejcikova ended bottom after losing all three matches.
The WTA Finals divides the top eight players into two groups of four, with each player competing in three matches. The top two from each group advance to the semi-finals.
"The first game was super long and I somehow lost my focus and of course I was a bit nervous to play a Czech woman," Pliskova said after the match.
"It wasn't easy at all, at some point I thought it is almost over in the second when she broke me to go up 4-2. I am proud that I kind of stayed there and fought for it."
Having never taken a set off Pliskova in two career meetings, Krejcikova quickly took care of that little bit of business with the French Open champion steamrolling through the opener in just 26 minutes.
Pliskova committed 12 unforced errors in the opening set but the former world number one steadied herself in the second. She still found herself in a hole when Krejcikova grabbed an early break on her way to a 4-2 lead.
With her opponent on the ropes, Krejcikova could not deliver the knockout blow as Pliskova dug deep sweeping the next four games to level the contest.
Pliskova's powerful serves and pinpoint forehands helped her fend off break points at 2-2 and 4-4 before she completed her comeback in a closely contested decider.
