Top seed Novak Djokovic started his bid for a record-equalling sixth ATP Finals title with a classy win over Argentine debutant Diego Schwartzman.
The 33-year-old Serb, who has not won the event since 2015, eased to a 6-3 6-1 victory at the O2 Arena in London.
"The second set I started swinging through the ball more and with less hesitation," Djokovic said. "I was very pleased with the way I played."
Daniil Medvedev and Alexander Zverev meet in the same group later on Monday.
The season-ending event is being played behind closed doors because of the coronavirus pandemic.