Roger Federer maintained his 100% record at this year's ATP World Tour Finals and ended Kei Nishikori's hopes with a three-set win in London.
The Swiss six-time champion came through 7-5 4-6 6-4 to claim his third round-robin victory at the O2 Arena.
Federer, 34, had already won the group and will be joined in the semis by Novak Djokovic or Tomas Berdych.
Top seed Djokovic just needs to take one set off Berdych when they meet at 20:00 GMT.
Federer forced to dig deep
The Swiss had qualified with victory over world number one Djokovic on Tuesday, but Nishikori still had a chance to progress and pushed the third seed to the limit in a match he did not need to win.