US Open 2013: Novak Djokovic outclasses debutant Joao Sousa to reach the fourth round in New York The world No 1 needed just 100 minutes to dispatch Sousa under the lights of the Arthur Ashe Stadium, setting up a fourth-round clash with Marcel Granollers of Spain, who upset 12th seed Tommy Haas in four sets. Djokovic was at his ruthless best against Sousa, who has impressed on his US Open debut but found himself out of his league against the Serbian. Within 28 minutes Djokovic was a set to the good and took the opportunity to work on his volleying, winning 22 of 30 points at the net. "I am not obviously as comfortable on the net as I am on the baseline, but I've been working on it," Djokovic said. It's part of my game that I still need to improve. I'm aware of that. It makes me happy that I have room for improvement, and I keep on spending hours and hours on the court and working on variety in my game - and the serve, especially that part of my game. I want to try and get as many free points on the serve as possible. Sousa, he world No. 95, had reached the third round at a Grand Slam for first time in his career, but the effort appeared to have taken its toll. The 24-year-old called for the trainer after the first set for treatment to his left thigh before finally winning his first game to level up at 1-1 the second set. That was an effort in itself, though, and his serve was broken again in the sixth game of the set as Djokovic reasserted his authority, cruising through the first five games of the third before a last hurrah from Sousa. The Portuguese held his serve and then earned huge cheers from the night session crowd for his first break of the match, only to have Djokovic seal his eighth break of the night to love to finish the contest.