Alexander Zverev beat Diego Schwartzman 6-3, 4-6, 6-3 on Wednesday to keep alive his hopes of another ATP Finals triumph in London.
The 2018 champion looked out of sorts in his straight-sets defeat against Daniil Medvedev on Monday and again struggled to find early rhythm at the empty O2 Arena.
The Argentine eighth seed, who is at the elite eight-man event in place of the injured Roger Federer, was also looking to bounce back from losing his opening round-robin match, to Novak Djokovic.
Big-serving Zverev, who struggled badly on second-serve points against Medvedev, was broken by Schwartzman in the third game but turned the tables with two breaks of his own to take the set.
The Argentine's serve was again under pressure in the first game of the second set but he dug himself out of the hole to hold before Zverev broke in the third game.
The German, who stands 28 centimetres (11 inches) taller than Schwartzman, appeared to be in control without finding his best form but then lost his own serve with a clutch of unforced errors.
Zverev raced through his next service game but then stumbled again on serve as his 28-year-old opponent took the set 6-4 to level the match.
The momentum appeared to be with the Argentine but the match changed direction again in the fifth game of the decider when Zverev broke with a fine backhand volley.
Schwartzman saved two break points in the seventh game but by now fifth seed was in the groove and he broke again to seal the win.
"It was much better than on Monday," said Zverev. "Diego is a very difficult player, he deserves to be here. There's no easy matches but I'm happy to get the win, I'm happy to give myself a chance to go to the semi-finals.
"I'm excited for Friday. Friday's going to be the most difficult match-up that you can have here, against Novak."
Zverev, who has won two titles in the coronavirus-truncated 2020 season, beat Djokovic in the final in London two years ago and appeared to be on the verge of a Grand Slam breakthrough.
The 23-year-old has not yet won a major but reached the semi-finals of the Australian Open this year and the final of the US Open.
On the eve of the tournament he opened up about domestic assault allegations, which he denies, but he came to London in good form after reaching the final of the Paris Masters, where he lost to Medvedev.
World number one Djokovic, seeking a record-equalling sixth ATP Finals title, is taking on Russia's Medvedev in the evening match at the season-ending championships.
Dominic Thiem is in charge of Group London 2020 after beating second seed Rafael Nadal on Tuesday to make it two wins out of two.
The O2 Arena is hosting the ATP Finals for the 12th and final season before championships move to Turin next year.
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