Sweltering conditions at the US Open are "no excuse" for a raft of early-round retirements, according to five-time Flushing Meadows champion Roger Federer.

A total of 14 players have retired across the men's and women's singles so far in the year's final grand slam, while fears were raised over players' safety on Thursday as American hope Jack Sock collapsed during his second-round meeting with Ruben Bemelmans.

Third seed Andy Murray complained that an illness affected him in testing conditions as he scraped into the third round with a 5-7 4-6 6-1 6-3 6-1 win over Frenchman Adrian Mannarino.

But Federer, who has sailed past Leonardo Mayer and Steve Darcis in straight sets so far, has little sympathy for those who have not adapted to the heat and humidity in New York.

"We've been here in North America for some time," Federer said.

"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.

"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."