Novak Djokovic has wished Stanislas Wawrinka a speedy recovery after the Swiss was forced to retire from their US Open last-16 match.

The defending champion held a 6-4 6-1 3-1 lead when his opponent bowed out due to an unspecified illness.

"I'm happy with my performances. I played well against a quality opponent and I wish him a quick recovery," said Djokovic.

The Serb, who held a 2-0 lead overnight when rain intervened, will face either Juan Martin del Potro or Andy Roddick in the quarter-finals.

"It was a difficult situation for both of us with the rain yesterday. We were here for 10 or 11 hours, warmed up and then stopped. It was hard to get ready for a match that way," he explained.

Djokovic emerged as US Open champion last year, defeating Rafael Nadal in the final.