Kei Nishikori beat Tomas Berdych to reignite hopes of progressing at the ATP World Tour Finals in London.
The eighth seed let an early lead slip and saved a break point in the decider to win 7-5 3-6 6-3 at the O2 Arena.
The Japanese had lost to Novak Djokovic in his opening round-robin match, while Czech Berdych lost to Roger Federer.
Djokovic and Federer meet at 20:00 GMT, with any victory certain to take Djokovic through and a straight-sets win enough for Federer.
Berdych recovered from an error-strewn start that left him a set and a break down to move within sight of victory with a break point at 3-3 in the decider.
"I think it's been all about one point in the third set when I had the break point," said the Czech sixth seed.
"I had him on the run. The rally starts pretty well for me, but I was not able to break him. Then I lost my serve. Basically, that was it. That was the difference."
Nishikori said: "That was a really big point. He had three, four shots that he went really aggressive.
"I was on defence all the time. I mean, the last shot I kind of went for it, closed my eyes. I was a little bit lucky I made that shot."