Fresh from his rest at the Paris Open, 6-time champion Roger Federer opened with a straight-set win over Jack Sock at the ATP Finals in London as he bids to end 2017 on a high.
The 36-year-old Swiss, making his 15th appearance at the season-ending event for the top eight players in the world, won 6-4, 7-6 at the O2 Arena.
American Sock was making his tournament debut, having qualified by winning the Paris Masters a week ago.
Alexander Zverev beat Marin Cilic 4-6, 6-3, 6-4 in Sunday’s second round-robin match in the Boris Becker Group.
