Dominic Thiem defeated defending champion Alexander Zverev to reach the final of the ATP Finals in London.
The Austrian, who had never reached the semi-finals in his previous three visits to the O2 Arena, won 7-5 6-3 in an hour and 34 minutes.
Thiem, 26, will now target a sixth title of a successful season against Greek Stefanos Tsitsipas, who earlier knocked out six-time champion Roger Federer, on Sunday.
The world No 5 stepped up in the decisive moments, just as he did in wins against Federer and Novak Djokovic during the round-robin stage, to seal a sixth win against Zverev from their eight meetings.
Thiem, who has struggled with flu this week, was under the most pressure on serve during the opening stages of the contest as he saved a break point in back-to-back service games.
By contrast, Zverev, who was aiming to become the first player to retain their title since Djokovic in 2015, was more dominant behind his serve as the first set appeared destined to be decided by a tiebreak.
But Zverev, 23, had begun to direct frustrations in the direction of his support camp and faltered in the 12th game of the set, when he double-faulted on Thiem’s second set point – his first break points of the match.
The German then vented his anger at the manner of losing the set by throwing his racket to the ground at the change of ends.
Thiem maintained his assurance into the second set and secured the break advantage at the end of an eventful sixth game, which featured a series of unforced errors from Zverev, on his third break point.
He then saved two break points, the second of which with a forehand winner, to consolidate the break and hurt Zverev’s hopes of an immediate route back into the match.
The 16-time winner on tour demonstrated his all-court ability to serve out for victory and set up the chance to become the first Austrian winner of the prestigious event.