Roger Federer says he's shocked by the record number of injury retirements at the US Open, suggesting some players could be taking the easy option by not finishing matches.
Czech ninth seed Tomas Berdych and Marcel Granollers, the Spanish 31st seed, took the Flushing Meadows injury toll to a Grand Slam record of 18 inside the first week when they quit their third round matches.
Former world No.1 Federer, who has only conceded a walkover once in a career which has seen him complete 973 matches, said some of his fellow professionals could be over-playing.
And he noted the strain of best-of-five sets grand slam tennis took a toll on niggling injuries.
"I'd say 50 per cent of them aren't lucky because (of) not feeling well or getting injured or carrying an injury," said Federer after reaching the fourth round with a 6-3 4-6 6-4 6-2 win over Croatia's 27th seed Marin Cilic.
"It depends where that player comes from. Maybe did they overplay a little bit? Has he been playing too long with an injury already?
"It comes out in best-of-five sets tennis. Can't hide it, in my opinion. Could some guys finish the matches? I'm sure, but they didn't decide to.
"For me it is shocking to see so many retirements. I have never retired in my whole life except once when I played against James Blake in Paris, but I didn't even walk on to the court.
"For me it doesn't matter how bad I'm feeling, I will be out there and giving it a try, because you never know what's going to happen.
"Every player feels different. It's unfortunate it happens for the fans, I guess."
Berdych was suffering from a right shoulder injury and needed treatment at the end of the first set against Serbia's Janko Tipsarevic, which he lost 6-4, and then called it quits when he was down 5-0 in the second.
Granollers, seeded for the first time at a major, was trailing Spanish compatriot and 2003 runner-up Juan Carlos Ferrero 6-1, 4-3 when he brought proceedings to a halt.
Tipsarevic and Ferrero will meet for a quarter-final place.
Berdych, 25, said he first felt the injury in his Cincinnati Masters quarter-final win over Federer two weeks ago before it caused him to quit his semi-final against Novak Djokovic.
"I already knew there was a problem. I had it in Cincinnati, so it was a case of just trying," he said.
Tipsarevic, who has reached the fourth round of the US Open for the first time, said he was keen not to ease off against Berdych despite his opponent's injury.
"I knew that if I became more and more dominant that he might think of retiring," said the Serb.
Two-time defending women's champion Kim Clijsters pulled out on the eve of the tournament suffering from a stomach muscle injury.
Former champion Venus Williams withdrew before her second round match with Sjogren's Syndrome, an auto-immune disorder.
In the men's draw, top seed Novak Djokovic benefitted in the first round when Irish qualifier Conor Niland retired through illness in the second set.
Defending champion Rafael Nadal reached the third round when French opponent Nicolas Mahut quit after two sets with a stomach strain.