Roger Federer brought Novak Djokovic's three-year unbeaten run indoors to an end to secure a semi-final place at the ATP World Tour Finals.
The Swiss six-time champion won 7-5 6-2, ensuring he will progress from the group stage with one match remaining.
Djokovic, winner of the last three titles in London, had not lost indoors since 2012, a run of 38 matches.
The Serb will play Tomas Berdych in his final group match on Thursday and can still qualify for the last four.
Federer will play Japan's Kei Nishikori, who beat Berdych 7-5 3-6 6-3 in the afternoon session.