Roger Federer of Switzerland celebrates at match point after beating Jack Sock of the United States in their singles tennis match at the ATP World Finals at the O2 Arena in London, Sunday, Nov. 12, 2017. (AP Photo/Kirsty Wigglesworth)
Federer has won a record six titles at the ATP Finals, including two in Houston, two in Shanghai and two in London. His last one came in 2011.
In 2017, he has won a tour-best seven titles, the most he has won since taking eight in 2007.
Sock is one of four players to qualify for the tournament for the first time. Zverev, Grigor Dimitrov and David Goffin also qualified, though Goffin played in the tournament last year as an alternate after Gael Monfils withdrew.
After the Federer match, Rafael Nadal was given his trophy for finishing the year as the No. 1 player in the rankings.
The Spaniard will play his first match at the tournament for the top eight players in the world on Monday against Goffin.