LONDON, Nov. 14 (Xinhua) -- World No. 1 Novak Djokovic continued with his winning start at the ATP Finals in the O2 Arena here on Wednesday when he defeated Alexander Zverev 6-4, 6-1 in an hour and 16 minutes.

Djokovic, who is attempting to win the trophy for a record equaling sixth time, was again very effective on his serve and although he only hit one ace in comparison to his rival's nine, he still won 75 percent of his first serves.

The first set was much more even than the second when Zverev was unable to cope with the Serb's relentless accuracy and the 21-year-old's game fall to pieces to give Djokovic a comfortable victory.

After previously beating the big-serving John Isner 6-4, 6-3 on Monday, Djokovic will qualify for the semifinals from the Guga Kuerton group if Isner beats Marin Cilic in Wednesday's second match or if Cilic defeats Isner in three sets.