A day after the ATP Finals lost Rafael Nadal to injury, Roger Federer lifted spirits at the O2 Arena by extending his stay to the weekend.
The 19-time Grand Slam champion guaranteed his place in Saturday's semifinals by defeating Alexander Zverev 7-6 (6), 5-7, 6-1 on Tuesday.
The top-ranked Nadal's withdrawal on Monday meant this was a clash between the two highest-ranked players left in the elite season-ending event, and it was predictably close.
Federer looked in control when he broke in the opening game of the second set, having claimed the first via a tiebreak. But Zverev produced two breaks of his own to level the match.
Undeterred by his 20-year-old opponent's persistence, Federer regrouped and broke three times in the decider to seal victory.
Zverev, who won his opening match against Marin Cilic, will face Jack Sock on Thursday to decide who will join Federer in the final four.
Sock recorded the United States' first victory at the event for 10 years by defeating Cilic 5-7, 6-2, 7-6 (4) in the early match, with the Croat later being eliminated by Federer's victory.
Sock, who seized qualification by winning the final Masters event of the season in Paris, lost his debut match to Federer, but he is now 1-1.
While others might have been surprised at Sock becoming the first American to win at the elite season-ending event since Andy Roddick in 2007, the man himself wasn't.
"I wouldn't be out here playing, competing if I didn't think I could give myself a chance to win tournaments, to be here," Sock said.
"I'm not the type of guy that will just enter a draw and be happy to make the third round or the quarters or whatever. I'm here to win matches. I think if I do the right things and I play the right tennis, I can give myself a chance to play on the weekend of any tournament."
Cilic has now won only one of his eight matches overall at the season-ending tournament. "Definitely not a great record, but I felt that I played even well last year," Cilic said. "In those three matches that I played, I had good chances at least to win. These two also. So just, you know, couple points away from closing those matches. A little bit unfortunate with that."