Andy Murray takes on Canadian fourth seed Milos Raonic in the semi-finals of the ATP World Tour Finals on Saturday.
The Scot, 29, is trying to win the season-ending finale for the first time, and end the year as world number one.
He must at least match the performance of Serbia's Novak Djokovic to retain top ranking.
Five-time champion Djokovic takes on Japanese fifth seed Kei Nishikori in the second semi-final at 20:00 GMT.
- Murray v Raonic is live on BBC Two and BBC Radio 5 live sports extra (online only) at 14:00 GMT.
Jamie Murray and Brazil's Bruno Soares will play in the second doubles semi-final at 18:00.
Murray has beaten Raonic seven times in a row, and five times this year, including in the Queen's Club and Wimbledon finals.
"It's a big match," said Murray, who is on a 22-match winning streak. "Milos serves big, goes for his shots. He moves forward when he has the chance.
"I think he probably likes the conditions here. It's a little bit quicker.
"You don't normally get loads of opportunities against the big servers. This year when I've played him, I've created a few opportunities in the matches.
"When they've come, I've been pretty clinical. I'll need to be the same tomorrow if I want to win."
Raonic, 25, has impressed so far in London, with his huge serve even more potent in the indoor arena, and he hopes the pressure of the rankings chase might finally get to Murray.
"I've played Andy many times this year. I haven't gotten the better of him," said the Canadian.
"I think the one thing that I might have is how much he has on his shoulders right now, a lot of consecutive matches, and as well what he's playing for.
"I have to try to do my best to try to accentuate that as much as possible in my own favour, so I can really get the most out of myself and hopefully be able to get on top of him for once this year."
Raonic was a doubt for the tournament after pulling out of the Paris Masters semi-finals with a leg injury, but the goal of ending the year as world number three has driven him on.
"It's very big for me," he said. "That's what I had for myself as the biggest motivator to come here and try to heal up quickly and play."
Djokovic goes into the weekend with a daunting record of 21 wins in his last 22 matches at the O2 Arena, and despite avoiding each other in the semi-finals, there remains the prospect of a tantalising final showdown with Murray on Sunday.