Rafael Nadal saved a match point to beat Russian Daniil Medvedev and boost his hopes of a first ATP Finals title.
Nadal, bidding to keep his world number one ranking, could have been eliminated on Wednesday with defeat but beat the fourth seed 6-7 (3-7) 6-3 7-6 (7-4).
The Spaniard trailed 5-1 in the third set and saved match point on his serve at 5-2 before a stunning comeback.
"This match is one of those that one [time] out of 1,000 you win," Nadal said. "Honestly, I was super lucky."
The 33-year-old's hopes of progression to the semi-finals rest on his meeting with Stefanos Tsitsipas on Friday.
Greek Tsitsipas plays Alexander Zverev, who beat Nadal in their opening round-robin match, later on Wednesday.