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 U.S. 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.

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