Roger Federer says he is surprised by the number of players who have withdrawn from the US Open due to the heat.
Jack Sock and Denis Istomin both retired from their respective second round matches on Thursday, taking the total number of tournament withdrawals to 14. Istomin's retirement, due to a non-heat related injury, marked a new Open era record for the highest number of men to pull out during a grand slam.
He was the 12th male player to retire at Flushing Meadows this year - so far, only two women have withdrawn.
"I'm surprised to hear that players are retiring because of heat. I mean, if you're injured, it's different and all that," Federer said following his victory over Steve Darcis. "I'm sure from the 12 or 13 players that have retired, I'm sure there's involvement with heat.
"What I don't understand, if that's the case, we've been here in North America for some time. It's not like, all of a sudden, hot. I mean, it was more on the warmer side, but it's not like impossible, to be quite honest. Really no excuse for that."
While the WTA employs a "heat rule", which allows players to request a 10-minute break before the start of a third set should temperatures and humidity rise beyond a certain point, the ATP has no such regulation.
The United States Tennis Association (USTA) has an "extreme weather policy" in place for WTA, junior and wheelchair players, which adopts a similar line to the heat rule. However, the USTA also follows ATP protocols, meaning male players are not entitled to a break.
That led to criticism from some sections of the men's tour; Stanislas Wawrinka was the latest player to question why the men are not entitled to a break, while Andy Murray revealed his heat and illness struggles during his five-set second round victory over Adrian Mannarino.
However, five-time US Open champion Federer, who faces 29th seed Philipp Kohlschreiber in the third round, remains baffled by the issue - and suggested fitness should not be an issue when it comes to the heat.
"I think everybody should be well-prepared," Federer added. "I know we don't play many best-of-five set matches all the time, so of course the body can react funny once you exceed the two-and-a-half to three-and-a-half hours of play.
"Mardy Fish, that's an exception. He's not been that well-prepared because he hasn't had the matches in his body. Other players, I don't know where they come from. So there are exceptions to those players.
"Maybe some guys already came in too tired, whatever it was. I think you have to analyse case by case. I think other players should be so fit that heat really shouldn't matter at that point, the ones we've been playing in."
Debate on the issue will continue as similarly hot conditions are expected in New York for the early parts of next week.