Japan's Kei Nishikori beat an out-of-sorts Stan Wawrinka in straight sets to take an early lead in his group at the ATP World Tour Finals.
Nishikori, seeded fifth, won 6-2 6-3 in the opening match of the John McEnroe Group at London's O2 Arena.
Swiss third seed Wawrinka made 31 unforced errors over the course of 67 minutes.
Andy Murray makes his debut as world number one when he takes on Marin Cilic in the evening session at 20:00 GMT.
While Murray and Novak Djokovic battle for the year-end number one spot, Nishikori took a significant step towards replacing Wawrinka as the world number three with a surprisingly easy win.