Dominic Thiem became the first player to advance from the group stage at the ATP Finals in London after outlasting five-time champion Novak Djokovic 6-7 (5-7) 6-3 7-6 (7-5).
Djokovic now faces a winner-takes-all match against Roger Federer to decide who will join Thiem in the semi-finals.
Federer beat Matteo Berrettini 7-6 (7-2) 6-3 earlier on Tuesday, which means he and Djokovic both have a 1-1 record before playing each other in their last round-robin match on Thursday.
Thiem improved to 2-0 while Berrettini was eliminated after a second loss.
A highly anticipated rematch of this year’s epic Wimbledon final, Djokovic and Federer’s clash was always going to be the highlight of the group stage at the season-ending tournament, even without the semi-final spot being on the line.
Federer also has the added incentive of avenging that Wimbledon defeat, when the Swiss star missed two match points before losing 13-12 (3) in the deciding set.
“Actually, it’s good for me to play him again, and maybe that all helps to get a chance to get him back,” Federer said.
“Maybe it took me a few days, couple weeks at most, to get over the Wimbledon loss.
“We’ll find out, but I think it’s all flushed away from my side. A lot of water has gone under the bridge since then.”
Federer has failed to reach the semi-finals only once in his previous 16 appearances at the ATP Finals, in 2008, while Djokovic is trying to equal the Swiss star’s record of six titles.
Thiem, for his part, has now beaten Djokovic in four of their past five meetings – although the previous three wins came on clay, his favourite surface.
On Wednesday, Rafael Nadal faces Daniil Medvedev in a rematch of this year’s US Open final, before defending champion Alexander Zverev plays Stefanos Tsitsipas in the late match.