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.
That has led to a flat and eerie atmosphere at the near 18,000-seat arena, meaning London has been unable to wave farewell to the event – which moves to Turin in Italy next year – like tournament organisers would have intended.