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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 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.
That has led to a flat and eerie atmosphere at the near 18,000-seater arena, meaning London has been unable to wave farewell to the event - which moves to Italian city Turin next year - like tournament organisers would have intended.
This is his 13th appearance in these finals in London and the lack of crowds was noticed by the 33-year-old.
He added: "I’ve been playing the ATP Finals in this arena for many years and have been blessed with some incredible matches and atmospheres.
"I must say it feels very strange to play in front of empty stands but for everyone, for all the tennis fans out there, we love you guys and miss you guys."
Diego Schwartzman gave his all against Novak Djokovic but the Serbian showed why he has the most wins ever at the ATP Tour Finals in London.