Andy Murray began life as the world number one with a convincing victory over Croatia's Marin Cilic at the ATP World Tour Finals in London.
The Briton, who replaced Novak Djokovic at the top of the rankings last week, won 6-3 6-2 at the 02 Arena for his 20th win in a row.
Murray and Djokovic are vying for the year-end number one spot.
Japan's Kei Nishikori beat Stan Wawrinka 6-2 6-3 in Monday's other John McEnroe Group match.
The evening session saw Murray step onto court as the world number one for the first time, and he delivered the result that most of the 17,000 spectators wanted.