Federer crashed out of the US Open in the early hours of Tuesday morning after a defeat in four sets to the relatively unheralded Australian John Millman and was not happy about the conditions.
Federer was below par and seemed to be suffering in the humid conditions, even though players are afforded a 10-minute break due to a change in rules introduced by Flushing Meadows officials.
Isner tumbled out of the tournament at the quarter-final round stage by Juan Martin del Potro.
And he thinks that if Federer has been complaining about the conditions, then there must be a problem.
When asked about the humidity, Isner said: “Yeah, look, I don’t think that’s a normal occurrence.
“It’s been pretty hot here, and it can be – I don’t think, whatever the humidity is on outer courts or in the city, I think it’s amplified on centre court.
“It’s just we have seen it all tournament long, the girls and the guys out there just sweating a lot.
“It’s very difficult. It’s not so much the heat, because the shade comes over the court, which is pretty nice, but it’s the humidity.
“It’s just very difficult to deal with.
“As you said, you know, I have never seen Roger sweat ever (smiling).
“If he’s sweating a lot and has to change clothes, then you know it’s pretty humid out there.”
Despite the defeat, Isner believes he has a good Grand Slam season overall.
He added: “No, definitely much better. It’s been a bugbear of mine for the last six years or so.
“Didn’t start off too great. I mean, I lost in the first round of Australia. My year didn’t start off great at all.
“But I have had a very good year, something that I’m proud of.
“But, no, it’s definitely something to build on.
“I’m still trying to build on things and learn from things that my experience at Wimbledon and here, two good results for me, back to back.
“Hopefully I can play like this more in the future and get back to a stage like this more in the future.”