Kei Nishikori remains on course for the ATP World Tour Finals last four after defeating David Ferrer in three sets in London.
The result means Andy Murray must beat the already-qualified Roger Federer in straight sets in the evening match to reach the semi-final and deny Nishikori a place in the knock-out phase in his first appearance at the season-ending tour finals.
Ferrer replaced Milos Raonic after the Canadian pulled out of his final round robin match with a quad tear, and won the opening set to confirm Federer's passage from Group B and leave Nishikori's hopes handing by a thread.
But the world No.5 won 12 of the next 17 games to prevail 4-6 6-4 6-1 but despite claiming a second victory his fate is out of his hands.
With Raonic having not confirmed his withdrawal until 90 minutes before his scheduled third round robin match against Nishikori, Ferrer had little time to prepare for his first match since being knocked out of the Paris Masters by the Japan player a fortnight ago.
But despite fears the world No.10 could be undercooked - and carrying a back injury which forced him to pull out of the Champions Tennis League - the Spaniard began in typically dogged fashion.
Nishikori did claim the first break to serve at 4-3 but Ferrer reeled off three successive games and converted the first of two set points as the US Open finalist netted a overhead volley.
When Federer took the first step it confirmed Federer's passage into the semi-final for a 12th time in 13 tournament appearances and was a dent to Nishikori's own hopes, with the 24-year-old now exposed to a group phase exit.
The No.4 seed immediately set about getting back into the match and broke Ferrer in the opening game of the second set before consolidating the advantage to lead 2-0.
And it was a lead Nishikori wouldn't allow to be wrestled away, denying Ferrer a single break-back chance as he squared the match by taking the first of two set points to keep his hopes alive of reaching the semi-final.
After Ferrer's brief resistance Nishikori had all the momentum at the start of the deciding set - the first to be played in the singles competition at this year's ATP Tour Finals - and he won the first four games before taking the match in 1 hour and 56 minutes, a victory that might still not be enough to qualify.