Serbian star Novak Djokovic played like the world No.1 he currently is, beating American John Isner 6-4, 6-3 to lead the Guga Kuerten Group of the Nitto ATP Finals at the O2 Arena here where he aims to win the tournament for the sixth time. With Cristiano Ronaldo as a witness in a VIP box, Djokovic beat Isner, the tallest player (2.08 m) in history to compete in the tournament in one hour and 13 minutes on Monday, reports Efe news. Isner, at 33 the third oldest newcomer in the history of the tournament, showed in the O2 Arena that his serve was his ultimate weapon with which he can destroy any opponent, except an inspired Djokovic. The American's serve marked a milestone this year at Wimbledon when he achieved a record 214 aces during his journey to the semi-finals, where he lost to South Africa's Kevin Anderson. But on Monday, Isner could not intimidate the Serbian star. The returns and shots of Djokovic's right staggered the giant from Greensboro (North Carolina) who suffered the ninth defeat in 11 confrontations.