Dominic Thiem recovered from squandering four match points in a dramatic second-set tie-break to beat Novak Djokovic in three pulsating sets in the last four at the ATP Finals on Saturday.
The Austrian third seed's 7-5, 6-7 (10/12), 7-6 (7/5) win ended Djokovic's hopes of equalling Roger Federer's record of six titles at the elite eight-man event.
The US Open champion will face either Rafael Nadal or Daniil Medvedev in Sunday's final as he chases the second-biggest title of his career.
The 2019 runner-up, who forced the only break in the first set, faced an uphill battle psychologically after his missed chances in the second-set tie-break.
But he was quickly back in the groove and held his nerve to rally from 4-0 down in the final-set tie-break, winning six successive points to earn two match points and taking the second of those.
"It was for sure a mental battle," said Thiem. "I got so tight in the second-set tie-break, first of all because to play these legends is always going to be something special.
"And then playing for a final here at the ATP Finals is also something very special. I thought after my first big title in New York maybe I'm going to be a little bit more calm but that was a mistake I guess.
"I was just as tight and nervous as before and it was so much on the edge, the match... I'm incredibly happy to be through."
Thiem took time to settle at the start of the match, hitting a number of unforced errors, but found his rhythm and his single-handed backhand became an important weapon as the match wore on.
The third seed produced a searing forehand winner to set up the first and only break point in the 11th game of the first set and Djokovic netted with a volley.
The 27-year-old closed out the set impressively, lashing a backhand winner down the line before producing an ace.
A frustrated-looking Djokovic was in trouble at break point down in the fifth game of the second set but Thiem netted with a forehand and the chance went begging.
Djokovic, 33, earned his first break point of the match in the eighth game but Thiem snuffed out the danger.
The Austrian, who lost last year's final at the O2 Arena to Stefanos Tsitsipas, produced a scrappy game when serving to stay in the second set but saved two break points against the 17-time Grand Slam champion.
In a nailbiting tie-break, Thiem squandered four match points and eventually dumped a backhand into the net as Djokovic took it 12-10 to level the match.
Both players settled quickly at the start of the decider, with Djokovic looking far more focused than in the latter stages of the second set.
The Serbian had a wobble in the eighth game but managed to hold and the match seemingly inevitably headed for a shootout.
Thiem looked down and out at 4-0 down in the final-set tie-break but rallied impressively, converting his sixth match point with a powerful forehand approach shot.
Thiem is only the second player in 2020 to win a tie-break against Djokovic, who entered the third-set decider with a 15-1 tie-break record this season.
The Austrian has lifted himself into the top echelon of the men's game, reaching the Australian Open final this year, where he lost to Djokovic, before his triumph at Flushing Meadows in September.
The world's top four players all made it through to the semi-finals for the first time at London's O2 Arena, which is hosting the season-ending event for the 12th and final year.
Second-seed Nadal, seeking his first title at the ATP Finals, plays world number four Medvedev in Saturday's evening match.
