Roger Federer has claimed that there is "no excuse" for players retiring due to the heat at the US Open.
Ten players retired after struggling with the hot conditions in the first round of the competition, while they were joined by United States' Jack Sock and Uzbekistan's Denis Istomin in the third round.
Federer has insisted that every player competing in the tournament should be fit enough to play through the heat.
"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," Federer told reporters.
"I think everybody should be well-prepared. 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.
"Maybe some guys already came in too tired, whatever it was. I think you have to analyse case by case. But I think other players should be so fit that heat really shouldn't matter at that point, the ones we've been playing in."