Germany's Alexander Zverev kept alive his hopes of reaching the ATP Finals playoffs after grinding past Argentine Diego Schwartzman on Wednesday.
Zverev, the sixth seed, recovered from a mini slump during the second set to defeat his opponent 6-3, 4-6, 6-3 in two hours and 11 minutes.
The 23-year-old, who won the ATP Finals title in 2018, will need to defeat world No 1 Novak Djokovic in his final group game on Friday to stay in contention for a place in the semi-finals.
While there were still a few lapses, Zverev ultimately had too much firepower for Schwartzman, the eighth seed who is making his debut in London in place of the injured Roger Federer.
Big-serving Zverev 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 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 the fifth seed was in the groove and he broke again to seal the win.
"That was much better than Monday, Diego is a very difficult player and he deserves to be here," Zverev said in his on-court interview after the match.
"There are no easy matches, so I'm happy to get the win and happy to give myself a chance to go to the semi-finals."
Zverev has an encouraging record against the 17-time Grand Slam champion, winning two of their five meetings, including in the ATP Finals title match in 2018.
"I'm excited for Friday," Zverev said. "It's going to be the most difficult match you can have here, against Novak. We've played twice here already, so I'm looking forward to it."
Zverev arrived in London having picked up a minor injury during his run to the Paris Masters final and he said his immediate focus was to recover his fitness levels ahead of his clash with Djokovic.
"I said from the beginning of the week that I think every match I'm going to improve because of that injury I had in Paris," Zverev said. "I needed to recover a little bit, I needed to get back into the rhythm and hopefully I can play even better on Friday."
Making his fourth appearance at the ATP Finals, Zverev has gathered plenty of experience at the season-ending tournament and its unique format, but he insists he is not relying on his past success in London to help him against Djokovic.
"Right now, it's just a regular tournament – I need to win every match otherwise I'm out, so I've got to go into it with that mindset, like I did today and I did my job, so hopefully I can do that again on Friday."
With the Group Tokyo 1970 action taking place on Wednesday, it's the turn of Group London 2020 on Thursday.
US Open champion Dominic Thiem takes on Russian seventh seed Andrey Rublev in the early evening match.
The third seed opened his campaign with a superb win over second seed Rafael Nadal, who needs to beat Greek fifth seed Stefanos Tsitsipas in the later match to avoid elimination.
For all of Nadal's success, the Spaniard has never won the ATP Finals title.