Rafael Nadal reached the ATP Finals last four by beating 2019 champion Stefanos Tsitsipas in three sets.
The Spanish world number two overcame Greek Tsitsipas 6-4 4-6 6-2 in London.
Nadal, 34, had been the more consistent player until losing serve to drop the second set, but fought back to dominate the decider.
The 20-time Grand Slam champion will play Russian Daniil Medvedev in the semi-finals at the O2 Arena on Saturday.
Medvedev is already guaranteed to win Group Tokyo 1970 with Novak Djokovic and Alexander Zverev contesting the remaining last-four place at 14:00 GMT on Friday.
That match is live on BBC Two and the BBC Sport website.
Earlier on Thursday, the already qualified Dominic Thiem lost 6-2 7-5 to Russian Andrey Rublev in a dead rubber.
US Open champion Thiem had reached the last four by beating Nadal and Tsitsipas to win Group London 2020.