New York - Defending champion Novak Djokovic of Serbia sailed into the quarter-finals of the US Open on Wednesday when fourth-round opponent Stanislas Wawrinka on Switzerland retired hurt.

Djokovic was in complete control of the match, leading 6-4, 6-1, 3-1 after an hour and a half on the Louis Armstrong Stadium court, when Wawrinka quit after getting medical help from his trainer at the change of ends.

Djokovic, who won the US Open for the first time last year, will play either American Andy Roddick or Argentine Juan Martin del Potro in Thursday's quarter-finals.

The world number two has not dropped a single set in the tournament so far. - Reuters