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.
In Monday's evening session, Russian fourth seed Daniil Medvedev earned his first ATP Finals match win by beating German fifth seed Alexander Zverev.
Medvedev lost all his group matches in 2019, but began with a 6-3 6-4 win.
The season-ending event is being played behind closed doors because of the coronavirus pandemic.