Rafael Nadal staged another comeback to stay in contention for a semifinal spot at the ATP Finals.
The top-ranked Spaniard still needs help from Daniil Medvedev to avoid an early exit.
Nadal rallied to beat already-qualified Stefanos Tsitsipas 6-7 (4), 6-4, 7-5 on Friday for his second win of the round-robin stage. That still won't be enough, though, if Alexander Zverev beats Medvedev in the last match of the group phase.
That would leave Zverev, Nadal and Tsitispas with identical 2-1 records. But Nadal would finish third in the group based on the tournament's tiebreaker rules.
Making things worse for Nadal is that his win ended Medvedev's chances of advancing, meaning the Russian (0-2) has only pride to play for against Zverev.
Nadal lost his opening match to Zverev and saved a match point at 5-1 down in the third set against Medvedev on Wednesday before rallying to win.
The comeback against Tsitsipas wasn't quite as dramatic. Nadal never faced a break point in the match but lost the last three points of the first-set tiebreaker to hand the Greek the lead.
But he broke for a 5-3 lead in the second set and again to make it 6-5 in the third, then converted his first match point when Tsitsipas netted a forehand.
Regardless of whether he advances, Nadal is guaranteed to leave London with the year-end No. 1 ranking after Novak Djokovic was eliminated in the group stage on Thursday with a loss to Roger Federer.
It's the fifth time that the 19-time Grand Slam winner ends the year atop the rankings, tied for second on the all-time list with Djokovic, Federer and Jimmy Connors. Pete Sampras did it six times.
At 33, Nadal is the oldest man to finish the year as No. 1.
However, he has never won the ATP Finals despite qualifying for a 15th year in a row. He has had to pull out of the tournament on six occasions because of injuries and reached the final only twice, the last time in 2013.