New York - 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 the 25-year-old Djokovic.
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 am on Monday morning, committed 53 unforced errors.
Later top seed Federer will take 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.
Latest results on Wednesday from the 10th day of the US Open (x denotes seeding):
Janko Tipsarevic (SRB x8) bt Philipp Kohlschreiber (GER x19) 6-3, 7-6 (7/5), 6-2
Novak Djokovic (SRB x2) bt Stanislas Wawrinka (SUI x18) 6-4, 6-1, 3-1 retired
Alsam-Ul-Haq Quereshi/Jean-Julien Rojer (PAK/NED x9) bt Chris Harrison/Ryan Harrison (USA) 6-2, 6-3
Sara Errani (ITA x10) bt Roberta Vinci (ITA x20) 6-2, 6-4
Maria Sharapova (RUS x3) bt Marion Bartoli (FRA x11) 3-6, 6-3, 6-4
Hsieh Su-Wei/Anabel Medina Garrigues (TPE/ESP x16) v Chuang Chia-Jung/Zhang Shuai (TPE/CHN) 6-2, 6-1
Andrea Hlavacova/Lucie Hrdadecka (CZE x3) bt Sabine Lisicki/Peng Shuai (GER/CHN) 6-3, 6-1
Nurla Llagostera Vives/Maria Jose Martinez Sanchez (ESP x8) bt Maria Kirilenko/Nadia Petrova (RUS x4) 7-5, 3-6, 7-6 (7/5)
Kveta Peschke/Marcin Matkowski (CZE/POL x4) bt Lisa Huber/Max Mirnyi (USA/BLR x1) 7-5, 6-2