Dominic Thiem demonstrated his power and resilience as he beat Novak Djokovic in three tough sets to reach the final in London.
For the fifth time in their last six meetings the pair could only be separated by a deciding set, and it was another astonishing clash as Thiem came through after plenty of perseverance.
The Austrian squandered four match points in an epic second set tie-break which he eventually ended up losing 12-10 as Djokovic demonstrated his trademark defensive efforts to somehow extend the encounter.
Incredibly, Thiem even contrived to double fault on one of his match points in the second set breaker, and his opponent was afforded the opportunity to remain in the match on a knife edge.
Having been pushed into a deciding set, the champion in New York may have been downhearted but he showed incredible poise and determination in taking victory after a second brutal tie-break.
Dominic Thiem of Austria celebrates during his match against Novak Djokovic of Serbia during Day 7 of the Nitto ATP World Tour Finals at The O2 Arena
Image credit: Getty Images
The second semi-final saw Daniil Medvedev, fresh from his triumph at the Paris Masters, come from behind to beat Roland Garros champion Rafael Nadal, who is still seeking the first title at the ATP Finals in his illustrious career.
Regardless of who now goes on to triumph in the last ATP Finals in London before the event moves to Turin next year, we are guaranteed a first-time champion for the fifth year in a row.
"It was a mental battle, I got so tight in the second-set tie-break," Thiem said.
"To play these legends is always special and to play 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."
Djokovic said: "What he did from 0-4 in the tie-break was unreal.
"I don't think I played bad, I made all my first serves; he just crashed the ball and everything went in. What can you do?"
Take up our Black Friday offer to make sure you don't miss a minute of Eurosport's tennis coverage in 2021