The third seed from Serbia lost the first set to the 15th-seeded Spaniard before battling back to win the fourth-round meeting 4-6 6-2 6-3 5-7 6-3 in an epic contest that lasted three hours and 44 minutes.

The first set went with serve until the ninth game, when a double fault from the Australian Open champion handed Robredo his first break-point of the match. The Spaniard did not pass up the opportunity, and went on to hold serve to go a set up.

After a strangely subdued opening, 21-year-old Djokovic burst into life at the start of the second set and soon built a 3-0 lead.

His momentum was halted when he had to call on the trainer to get treatment, seemingly for a hip problem, but was soon able to resume and - after each player held serve twice - capitalised on a double fault by Robredo to break again and level the match at one set all.

Sensing the Girona-born 26-year-old may be running out of steam, Djokovic broke Robredo in his second service game of the third set to establish a 3-1 advantage.

Robredo, making his fifth appearance in the fourth round, was looking to make the last eight at Flushing Meadows for the first time but could not find a way back with the remainder of the set going with serve.

Djokovic was then forced to serve to stay in the fourth set at 5-6 and failed to hold with Robredo breaking to take the match into a decider.

But the world number three responded well to that setback, breaking Robredo in the fifth and ninth games to finally wrap up the win.