Serbia's Novak Djokovic rolled to an easy 6-2, 6-1 win on Sunday over Italy's Matteo Berrettini in his Group Bjorn Borg debut at the ATP Finals in London.

"Throughout my career, I always intended and always tried to perfect my game and in the midst of a battle and heat of the moment, sometimes you just get frustrated with certain things regardless of the results," Djokovic said. "Sometimes I really don't mind the results as much as I care about the execution of certain shots or tactics or whatever it is."