Alexander Zverev shocked Novak Djokovic to set up a Nitto ATP Finals decider against defending champion Daniil Medvedev in Turin.
Djokovic had breezed through to the last four without dropping a set as he chased a record-equalling sixth title and a first for six years.
But third seed Zverev, who ended the world number one's hopes of a Golden Slam by beating him at the Olympics in Tokyo, played a superb match to win 7-6 (4) 4-6 6-3.
On Sunday, the 2018 champion will take on last year's winner Medvedev, who had a straightforward 6-4 6-2 victory against Norwegian debutant Casper Ruud.
Medvedev defeated Zverev in the group stages in a match that went right to the wire and will be the favourite having won their last five meetings.
Zverev knows he has the weapons to beat Djokovic and that confidence shone through as he saved a set point at 5-6 in the opener before playing an exceptional tie-break.
Rarely can Djokovic have lost a set where he has played at such a high level, and he seemed rattled in the early stages of the second but responded in trademark fashion to level the match.
A sloppy game early in the third was uncharacteristic, though, and Zverev held firm on his own serve to clinch a fourth career win over the Serbian.
