Andy Murray came through the longest match in ATP World Tour Finals history as he saw off a resilient Milos Raonic to set up a potential final with Novak Djokovic that would decide the year-end world number one ranking.
Murray, who displaced Djokovic at the top of the rankings this month when Raonic withdrew from their semi-final at the Paris Masters, had already broken the record for the longest match at tournament since detailed records began in 1981, in an epic three-set battle in the round-robin stage with Kei Nishikori that lasted three hours and 20 minutes.
And he needed to summon all of his reserve yet again, requiring 18 further minutes to come from a set down to defeat Raonic 5-7 7-6 (7-5) 7-6 (11-9).
The victory means a Djokovic win over Nishikori in the second semi-final would ensure a mouth-watering winner-takes-all decider at the O2 Arena on Sunday.
Murray saved four break points in the first set before the pressure told in the 11th game, as the Briton double-faulted and Raonic subsequently held to get his nose in front.
Facing the man he defeated in the final at the All England Club in July to become Wimbledon champion for the second time, Murray sent a forehand low into the net to go a break down at 2-1 in the second but hit straight back and broke to love courtesy of a powerful forehand that Raonic did well to get a racquet on.
A further two break points went begging for Murray as Raonic held to keep the set on serve and the Scot looked to have wasted a further opportunity in the eventual tie-break.
His 4-1 lead was quickly eroded, Raonic fighting back to level at 5-5, only for Murray to force a decider with an excellent volley and then a serve the world number four could only fire long.
Murray's serve came under intense pressure as he fell 0-30 down when trailing 3-4, however, he recovered to salvage a hold with an ace and then pounced on an error-strewn service game from Raonic, who handed him a break to love with a cross-court backhand into the net.
Raonic refused to surrender lightly and instantly carved out two break-back points, snatching the second courtesy of some marvellous play at the net.
He then repeated the feat after again surrendering the initiative to Murray to set up a second successive shoot-out.
Once more Raonic's resilience was tested to the limit, saving three match points and failing to take one of his own as Murray rescued the situation with a calm volley.
A pulsating contest was finally brought to a close when Raonic netted a forehand on the run. The only question now is whether Murray has enough left in the tank for his maiden final at the tournament as he seeks to end the year at the summit of men's tennis.
Murray  bt Raonic  5-7 7-6 (7-5) 7-6 (11-9)
Murray - 27/36
Murray - 7
Raonic - 10
BREAK POINTS WON
Murray - 3/7
Raonic - 4/12
FIRST SERVE PERCENTAGE
Murray - 61
Raonic - 63
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Murray - 62/63
Raonic - 74/45
Murray - 138
Raonic - 136