Andy Murray is into his first season-ending final and on course to end the year as world number one after beating Milos Raonic at the ATP World Tour Finals.
The Briton, 29, won a gripping semi-final 5-7 7-6 (7-5) 7-6 (11-9) in three hours and 38 minutes.
Murray must match Novak Djokovic’s results at the O2 Arena to stay ahead of the Serb in the rankings.
Djokovic plays Japanese fifth seed Kei Nishikori in their semi at 20:00 GMT.
The victory over Canadian fourth seed Raonic extended Murray’s winning streak to a career-best 23 matches, stretching back to September.
It was the Scot’s second lengthy encounter in four days, breaking the tournament record of three hours and 20 minutes he took to beat Nishikori in the round-robin stage on Wednesday.
“I didn’t expect to play a long one with someone with a serve like Milos,” said Murray.
“I’m tired. I’ve played so much tennis over the last few months. I’ll give it my best effort – the best of what I have.”
Raonic tests Murray to the limit
Murray was yelling at himself and 17,000 spectators were screaming with every twist and turn before he finally prevailed in an extraordinary contest.
He twice failed to serve out the match as the clock ticked past three hours in the final set, and three match points then slipped by in the tie-break.
Raonic had lost his past seven matches against Murray, including the finals at Queen’s Club and Wimbledon this year, but when his chance came with a match point of his own it was snuffed out by a sharp Murray volley.
Finally, at the fourth time of asking, Murray converted a match point to reach his first final at the O2 Arena and remain in control of his number one hopes.
“It was an amazing atmosphere,” added Murray.
“The longer the match went on, the louder the crowd got. This is what we play for. Matches like this and arenas like this. This is one of hardest matches I’ve played indoors.”
How did Murray fight back?
Raonic took the first set after Murray’s second serve had kept him in it for almost an hour, only to double fault on a fourth break point at 5-5.
An early break in the second had the Canadian within sight of victory, but only briefly as he followed up with an error-strewn game to hand the advantage back at 2-2.
Murray was two points from defeat in the tie-break when he was pegged back to 5-5 but a gorgeous forehand volley edged him ahead again, and a serve levelled the scores.
It seemed he had finally done enough with a break at 4-4 in the decider, but a pumped-up Raonic thumped away a smash to break back, and two games later he repeated the trick when Murray netted a forehand.
But with his knuckles bloodied from angrily punching his racquet rings, Murray would not yield.
The Wimbledon champion came through a second nerve-jangling tie-break to secure his place in Sunday’s final (18:00 GMT, BBC Two), and a chance to make history once again.