Defending champion Novak Djokovic finally made it through to the quarter-finals of the US Open after Stanislas Wawrinka retired in the third set of their match at Flushing Meadows.

The pair had begun their fourth-round clash on Tuesday night before rain brought a premature end to play. They resumed just after 1pm on Wednesday.

Djokovic was tested in the first set but after that it was a one-sided contest and Wawrinka called it a day with the second seed leading 6-4 6-1 3-1, although the reason was not immediately apparent.

They managed only two games before the rain came on Tuesday, Djokovic winning both of them, but Wawrinka quickly retrieved the break on the resumption.

The last time the Swiss beat Djokovic was six years ago and his cause was not helped when he promptly dropped his serve again.

Djokovic has cruised through the draw in New York but he was making more unforced errors than usual on a sparsely-populated Louis Armstrong Stadium and was unable to serve out the set.

However, for the second time Wawrinka failed to hold serve straight after breaking and things swiftly went downhill in the second set for the 18th seed.

After saving a break point in the opening game, Djokovic reeled off four games in a row, and Wawrinka responded by smashing his racquet on the court.

It did not help, though, and he was quickly a break down in the third set. Wawrinka talked to the trainer after the change of ends and, following the fourth game, he called Djokovic to the net and shook hands.