Italian top seed Jannik Sinner defeated American Taylor Fritz 6-4 6-4 to move closer to the last four of the ATP Tour Finals.
The 23-year-old world number one fought off stubborn resistance for his second win in front of a home crowd in Turin.
Sinner, who beat Fritz in September's US Open final, hit 22 winners to Fritz's 19 as he edged a compelling encounter.
Earlier, a rejuvenated Daniil Medvedev defeated Alex de Minaur 6-2 6-4 in the Ilie Nastase Group.
Fourth seed Medvedev was irate during his opening defeat by Fritz on Sunday but was far more settled in Tuesday's victory.
Meanwhile, Britain's Henry Patten and partner Harri Heliovaara are through to the doubles semi-finals after a second straight triumph.
Sinner, who also took the 2024 Australian Open, tops the group table with one round to go as he seeks another big tournament triumph in a year where he has also been embroiled in an ongoing doping controversy.
The winner of seven tour titles in 2024, he was claiming his 67th win from 73 matches - cheered on by a capacity crowd of 12,000 which included Italian football great Andrea Pirlo.
"It was a very tough match - we got to know each other very well in a Grand Slam final," said Sinner.
"The crowd is helping me a lot, and it's a big occasion for me."
Fritz had earned the match's first break point in the seventh game but could not make it count.
The American fifth seed fought back from 0-40 behind in the next to level at 4-4 before succumbing on his following service game as Sinner piled on the pressure to clinch the first set.
A similarly tight second set followed and again Sinner stepped up to shatter his opponent's resolve, as typified by a breathtaking backhand down-the-line winner en route to going 4-3 ahead on serve.
He went on to gain the crucial break, courtesy of a Fritz double fault, for a hard-fought but deserved victory.
'I blocked the noise' - Medvedev
Medvedev put Sunday's frustrating defeat behind him
Russian Medvedev, who had been booed for his antics against Fritz, put his fingers in his ears after sealing victory against De Minaur.
He lost only nine points on his serve as he wrapped up the win in an hour and 18 minutes.
"I went into this match blocking the noise, even from myself," said 2020 champion Medvedev, 28.
"I really didn't care what was happening on the court, I just tried to play, and it was a good feeling."
Medvedev's temperament was under scrutiny after his surly behaviour 48 hours earlier but he struck the ball superbly and broke in the third game with a cross-court winner.
He gained a double break when De Minaur double-faulted, and went on to take the first set in 40 minutes.
De Minaur, 25, battled in a pulsating second set but Medvedev's stunning backhand put him 5-4 ahead and he held serve to clinch victory.
In the doubles, Patten and Heliovaara beat Australians Max Purcell and Jordan Thompson 7-6 (7-3) 7-5 in a repeat outcome to their Wimbledon final.
The British-Finnish duo are bidding to become the first team to capture the Wimbledon and ATP Finals trophy in the same season since Mike Bryan and Jack Sock in 2018.
Alcaraz abandons practice session
Carlos Alcaraz lost 6-1 7-5 to Casper Ruud on Monday
French Open and Wimbledon champion Carlos Alcaraz's prospects in the event are uncertain after he pulled out of practice.
His coach Juan Carlos Ferrero told Spanish media the 21-year-old cut short his session after just five minutes as a chest complaint was making breathing difficult.
The Spaniard is due to play Russian Andrey Rublev in the John Newcombe Group on Wednesday (13:00 GMT).
"I don't think it will get to the point of not playing, but it will be difficult for him to be at 100% tomorrow," said Ferrero.
Alcaraz was out of sorts when losing his first match in Turin on Monday to Norwegian Casper Ruud, who next faces German Alexander Zverev in Wednesday's evening session.
BBC