Rafael Nadal began his bid for the biggest title missing from his vast collection with an ATP Finals win over Russian debutant Andrey Rublev.
Spanish second seed Nadal, 34, won 6-3 6-4 as the season-ending event began in London for the final time.
Earlier on the opening day, third seed Dominic Thiem avenged his defeat by Greece's Stefanos Tsitsipas in last year's final with a three-set win.
US Open champion Thiem won 7-6 (7-5) 4-6 6-3 at an empty O2 Arena.
Nadal's straight-set win moved him above the Austrian at the top of Group London 2020 - named in tribute to the event's 50th anniversary - after the opening matches.