World number two Novak Djokovic saw off big-serving Milos Raonic in a pulsating contest to reach the semi-finals at the ATP World Tour Finals.
The five-time champion won 7-6 (8-6) 7-6 (7-5) at the O2 Arena to become the first man through to the last four.
The win means the Serb, who beat Dominic Thiem on Sunday, will regain the number one ranking from Briton Andy Murray if he goes on to take the title.
Canada's Raonic faces Austria's Thiem for the other semi-final place on Thursday.
Thiem beat Frenchman Gael Monfils 6-3 1-6 6-4 in Tuesday's opening match in the Ivan Lendl Group.