Novak Djokovic began his bid for a record fourth straight end-of-season title with victory over Japan's Kei Nishikori at the ATP World Tour Finals.
The Serb won 6-1 6-1 to claim his 15th consecutive win at London's O2 Arena, a run stretching back to 2011.
Roger Federer beat Tomas Berdych 6-4 6-2 in the opening day's other round-robin match.
Britain's Jamie Murray won on his debut as he and doubles partner John Peers beat Simone Bolelli and Fabio Fognini.
All the players wore black ribbons on their shirts as a mark of respect to the victims of Friday's deadly attacks in Paris.