Austrian Dominic Thiem became the first player to reach the semifinals at ATP World Tour Finals after clinching a sensational win over world number two Novak Djokovic here on Tuesday.

Thiem stunned Swiss legend Roger Federer in straight sets last Sunday in his first match in Group Bjorn Borg, then came from behind to edge in-form Djokovic 6-7 (5), 6-3, 7-6 (5) this evening.

“This was really one of these special matches, what I’ve practiced all my life for, all my childhood for,” said the 26-year-old Thiem, who reached the semifinals of the season finale for the first time after qualifying for the tournament four years in a row.

Earlier, Swiss legend Federer defeated tournament debutant Matteo Berrettini from Italy 7-6 (2), 6-3. As the six-time champion lost his first match to Thiem, he has to defeat Djokovic on Thursday to reach the semifinals.

The last round group match will be the 49th encounter between the two prolific players. Djokovic beat Federer in this year’s Wimbledon final in a five-set thriller and leads the head-to-head 26-22.