Austrian third seed Dominic Thiem avenged his defeat by Greece's Stefanos Tsitsipas in last year's final with a three-set win as the ATP Finals began in London for the final time.
US Open champion Thiem won 7-6 (7-5) 4-6 6-3 in the opening singles match at the annual season-ending event.
Thiem upped the tempo in the decider, hitting 11 winners to see off sixth seed Tsitsipas at an empty O2 Arena.
In the same group, Spain's Rafael Nadal plays Russian debutant Andrey Rublev.
The pair meet in Sunday's evening session looking to match Thiem's opening win in Group London 2020.