Japan's Kei Nishikori surged to a three-set victory over alternate David Ferrer at the ATP World Tour Finals and must now wait anxiously to see if he has done enough to reach the semi-finals.
The 24-year-old lost the opening set to the gritty Spaniard, a late stand-in for injured Milos Raonic, but played beautifully after that to seal a 4-6, 6-4, 6-1 win - his second in Group B after also beating Andy Murray.
The result was enough to guarantee six-time champion Roger Federer a place in the semi-finals even before the Swiss walks on court to face home favourite Murray in the evening session.
Should Murray win in straight sets he would top the group, but any other result would send Federer through as group winner with Nishikori runner-up and the Scot packing his bags.
Earlier, Canada's Milos Raonic withdrew from the tournament due to injury and was replaced by Ferrer.
The 23-year-old lost his first two round-robin matches at the season-ending tournament to Federer and Murray although still had an outside chance of reaching the semis.
"I am sad to announce that I have to withdraw from the rest of this competition, due to a low grade tear in my quad. It was a hard decision," Raonic said. "You, the fans deserve a great show, and I wouldn't step out if I couldn't. I am sorry for the fans. I will do everything to be here next year."