Dominic Thiem came back from the brink of defeat to beat Novak Djokovic in a gripping last-four clash at the ATP Finals on Saturday as he reached the final for the second successive year.
The 27-year-old Austrian appeared to be sliding to defeat when he trailed 0-4 in the third-set tiebreak but stormed back to claim a 7-5, 6-7(10), 7-6(5) victory in just under three hours.
World No. 3 Thiem was the better player in the first set, sealing it with an ace after breaking serve at 5-5.
An absorbing second-set tiebreak saw Thiem get four chances to seal victory but he double-faulted on one match point and Djokovic showed true grit to snatch the set.
Serves dominated in the decider and when Thiem started the day's second tiebreaker and then lost the next three points it seemed as though the end was nigh.
Instead the world number three roared back, winning the next six points.
Djokovic saved a fifth match point but Thiem finally got the job done when a Djokovic groundstroke drifted long.
Thiem will now try to complete a remarkable year, in which he claimed his first Grand Slam title by winning the U.S. Open, against either Rafa Nadal or Daniil Medvedev in Sunday's climax.
"It was a mental battle, I got so tight in the second-set tiebreak," Thiem, who avenged his defeat by Djokovic in this year's Australian Open final, said on court.
"To play these legends is always special and to player here is also something special. I thought after winning the title in New York I would be more calm, but that was a mistake I guess." Thiem will attempt to go one better than last year when he lost to Stefanos Tsitsipas in the final.
Defeat means Djokovic will have to wait until the event moves to Turin next year, after 12 years in London, to try again to equal Roger Federer's six titles at the year-ender.