| NEW YORK
NEW YORK Defending champion Novak Djokovic of Serbia sailed into the quarter-finals of the U.S. Open on Wednesday when his fourth-round opponent Stanislas Wawrinka on Switzerland quit in the third set.
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 said he could not go on after getting medical help from his trainer at the change of ends.
"I really don't know exactly what it was but by the look of it, I think it was probably a dizziness or something," said Djokovic.
"He served well, he played well, but you could see that he didn't feel great on the court."
Djokovic, who won the U.S. Open for the first time last year, will play Juan Martin del Potro in Thursday's quarter-finals after the Argentine beat retiring American Andy Roddick 6-7 7-6 6-2 6-4.
The world number two has not dropped a set in the tournament so far and is pleased with the way he is playing.
"It was really important for me to get the first set under my belt so I could get that mental advantage," he said.
"I think I have done well in important moments I served well.
"It's great that I haven't spent that much time on the court. I played well, and I'm looking forward to the quarter-finals."
Djokovic, however, could not hide his frustration at the length of time the match took to play. It started on Tuesday but was one of five matches held over until Wednesday because of rain.
In each of the last four years the men's final was delayed until the Monday due to foul weather, triggering an annual debate and complaints from players, including Djokovic, over why the main court was not covered.
"I really hope that they will reconsider in this event, and any other hard court event, to at least use covers if the roof is no option," he said.
"It doesn't change my opinion about this tournament. I still love playing here.
"But I just think that considering the experiences that we all had with weather in last four, five years, it's the most logical explanation and solution you can come up with."
(Editing by Frank Pingue/Greg Stutchbury)