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.
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 on Friday.
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