Surprised by number of heat retirements: Federer

Seventeen-time Grand Slam winner Roger Federer said he has been surprised by the record number of players who have had to retire because of heat at the ongoing 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, there is no excuse. We have been here in North America for some time," Federer was quoted as saying by bbc.com on Friday.

Federer, 34, beat Belgian Steve Darcis 6-1, 6-2, 6-1 to reach third round.

The five-time US Open champion will next face Germany's 29th seed Philipp Kohlschreiber.

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

"I know we do not 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," the 34 year-old said.

"I think you have to analyse case by case. But other players should be so fit that heat really should not matter at that point," Federer said.