A rejuvenated Daniil Medvedev defeated Alex de Minaur 6-2 6-4 in his second match at the ATP Tour Finals.
Fourth seed Medvedev was irate during his opening defeat by American Taylor Fritz on Sunday but looked far more settled in Tuesday's victory.
The 28-year-old Russian, who had been booed for his antics against Fritz, put his fingers in his ears after sealing victory,
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.
"I really didn’t care what was happening on the court, I just tried to play, and it was a good feeling.
"The more popular you become, the more fans you get, the more haters you get, the more attention you get. Sometimes even the good noise can make you off balance.
"You win everything and people say you’re a god. You lose two matches and people say 'Your career is finished.' Sometimes it’s good to just block it."
Australian De Minaur will be knocked out with a match to spare if the other tie in the Ilie Nastase Group between Italian top seed Jannik Sinner and Fritz goes to three sets later on Tuesday (19:30 GMT).
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.
Meanwhile, Britain’s Henry Patten and partner Harri Helioevaara are through to the doubles semi-finals after a second straight triumph.
They 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.
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.