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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 Medvedev, I found the right shots at the right time. I have tremendous respect for Sascha, a great player and not easy to return 140mph first serves.
"It was anybody's game for most of the match."
Djokovic is bidding to equal Roger Federer's record of six ATP Finals titles, while Nadal seeks a first.