US Open 2015: No excuse for heat retirements - Roger Federer

Roger Federer will face Germany's Philipp Kohlschreiber in the US Open third round

Roger Federer says he has been surprised by the record number of players who have had to retire because of heat at the US Open.

Jack Sock and Denis Istomin both withdrew, taking the tournament total to 14 retirements - equalling the Grand Slam record after two rounds.

"I think everybody should be well-prepared. We've been here in North America for some time," said Federer.

"It was more on the warmer side, but it's not impossible, to be honest."

Federer, 34, beat Belgian Steve Darcis to reach round three.

The five-time US Open champion will face Germany's 29th seed Philipp Kohlschreiber in the next round,

Despite temperatures reaching 33C, the Swiss believes players should be fit enough to cope with the conditions.

He 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.

"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."