Defending champion Novak Djokovic reached his 14th successive Grand Slam quarter-final on Wednesday when Swiss 18th seed Stanislas Wawrinka was forced to retire from their US Open last-16 match.

Serb second seed Djokovic was 6-4, 6-1, 3-1 ahead when Wawrinka, who had already complained of feeling unwell at the end of the third game of the third set, called it quits.

Djokovic, who was leading 2-0 overnight when the match was halted because of rain, goes on to face either 2009 champion Juan Martin Del Potro, the Argentine seventh seed, or 2003 winner Andy Roddick for a place in the semi-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," said Djokovic, the 25-year-old Australian Open champion.

Djokovic has reached his sixth successive US Open quarter-final by dropping just 20 games in four rounds.

"I'm happy with my performances. I played well against a quality opponent and I wish him a quick recovery," added the Serb, who played his match in front of just a few hundred fans on the 10,000-capacity Louis Armstrong Stadium.

Wawrinka, who was bidding to reach the quarter-finals for the second time, having made the last eight in 2010, has now lost 19 matches in a row against top-10 players.

Djokovic's fellow Serb, eighth seed Janko Tipsarevic, fresh from a Twitter stir where he reignited the equal prize money row, reached his second successive US Open quarter-final.

Tipsarevic brushed past German 19th seed Philipp Kohlschreiber 6-3, 7-6 (7/5), 6-2 and goes on to face Spanish fourth seed David Ferrer for a place in the semi-finals.

Just days after criticising the entertainment value of women's tennis, and questioning why the likes of Maria Sharapova and Serena Williams didn't play five sets, Tipsarevic played in front of just a smattering of fans on the Grandstand.

The 28-year-old, who led 5-2 in the first set overnight before his fourth-round match also fell victim to the heavy New York rain, enjoyed his best Grand Slam performance at the US Open in 2011 when he reached the last eight.

But a knee injury forced him to retire in the fourth set against Djokovic.

Kohlschreiber, whose previous match against John Isner ended at a record-equalling late finish of 2:26 a.m. on Monday morning, committed 53 unforced errors.

Roddick, 30, and playing his last match before retirement, was just a point ahead in the first set tie-break against Del Potro when play was halted overnight.

Del Potro -- whose New York triumph victory in 2009 was the only time in the last 30 majors that either Djokovic, Roger Federer or Rafael Nadal wasn't champion -- is trying to make the last eight for the first time since that win.

Roddick's 2003 victory in New York remains the last time an American man won a Grand Slam title.

On a bumper day of action at Flushing Meadows, top seed Federer was taking on Czech sixth seed Tomas Berdych for a place in the semi-finals.

Federer, bidding to become the first six-time champion in 87 years and make a ninth straight semi-final in New York, will meet either British third seed Andy Murray or Marin Cilic, the 12th-seeded Croat, for a place in the final.

Federer beat Murray in the 2008 US Open final.