Andy Murray takes on Milos Raonic on Tuesday evening knowing his ATP World Tour Finals hopes are on the line.
The Briton lost his opening Group B match in straight sets against Kei Nishikori and now faces a battle to qualify for the semi-finals.
Roger Federer, who beat Raonic on Sunday, will play Nishikori in Tuesday's afternoon session in London.
Murray is likely to require two victories and it could come down to how many sets or games have been won.
"I'm going to have to win those two matches and try to win them well if I can," Murray said. "It's going to be tough and I'll need to play better than I did on Sunday.
"But that's the nice thing about the round-robin - if this was another tournament I'd be going home, whereas here at least I have another opportunity to try and get back in the tournament."
Two players will progress from the group, with Murray up against Federer in his third match on Thursday, and the Swiss six-time champion looked in good form when he beat Raonic on Sunday.
The 23-year-old Canadian is making his debut at the season finale but had expected more of himself having beaten Federer for the first time in Paris two weeks ago.
"I get pretty angry when I lose, so I'm going to have to learn how to slap myself out of it," he said.
Raonic rose from 11th in the world at the start of 2014 to a high of sixth off the back of his Wimbledon semi-final run in July, and held his position in the top eight during recent weeks to secure his place in London.
He also has a 3-1 winning record against Murray, including victory in theironly meeting this year in Indian Wells.
"It's a completely different match," Raonic said. "Things are going to be quite different than the first match [in London].
"We both have more of an idea what we need to do different for the next round.
"It's going to be about who adjusts better, who can play better come Tuesday night."