Novak Djokovic moved past Alexander Zverev in straight sets to reach the last four of the ATP Finals.
The Serb world number one beat German Zverev 6-3 7-6 (7-4) at the O2 Arena in London.
Djokovic, seeking a record-equalling sixth title at the end-of-season tournament, was pushed much more in the second set but raised his level at the crucial time in the tie-break.
He will play Dominic Thiem in the last four on Saturday at 14:00 GMT.
US Open champion Thiem progressed as winner of Group London 2020 with Djokovic going through in second position in Group Tokyo 1970.
The other semi-final is between Rafael Nadal and Daniil Medvedev, who first plays a dead rubber against Diego Schwartzman at 20:00 GMT on Friday.
“I felt great. In the first set he had a couple of break-point chances and I managed to serve well in the important moments,” said 33-year-old Djokovic.
“In contrast to the match against Daniil, I found the right shots at the right time. I have tremendous respect for Sascha, who is a great player and it is not easy to return 140mph first serves.
“It was anybody’s game for most of the match.”