NEW YORK (AP) — Roger Federer positioned a tiny black fan so it would blow air right at his face during changeovers in a bid to cool off during what became a stunning loss at the U.S. Open.

Rafael Nadal piled up so many soaked white towels next to his sideline bench the following night that it looked like laundry day. The man he beat after five sets and nearly five hours, Dominic Thiem, found it impossible to run in shoes he called "completely wet."

And a day later, Novak Djokovic's quarterfinal opponent made an unusual plea to leave the court at 2-all, right in the middle of a set, so he could change out of his drenched clothes and sneakers — and Djokovic was OK with it, because it gave him a chance to remove his shirt and chill out for a bit.

Djokovic leaned back, put his hands behind his head and stretched his legs Wednesday night in Arthur Ashe Stadium, looking more like someone relaxing by the pool than a guy in the grind of a Grand Slam match.

Don't sweat the small stuff? Try telling that to Federer, Nadal, Djokovic and all of the other players, ball boys and girls, spectators and everyone else dripping through this most uncomfortable of U.S. Opens, where it's not just the heat, it's the humidity.

"I personally have never sweat as much as I have here. Incredible. I mean, I have to take at least 10 shirts for every match," said Djokovic, a two-time champion at Flushing Meadows who faces 2014 runner-up Kei Nishikori in the semifinals Friday. "It's, literally, after two games, you're soaking."

He was among those who noted just how much even the stoic Federer — gasp! — was perspiring when he was upset in the fourth round on Monday night, when the humidity was about 75 percent. It's exceeded that on other evenings, while the temperature keeps topping 90 degrees (32 Celsius), day after day, although it's supposed to begin dropping Friday.

"I have never seen Roger sweat. Ever," said John Isner, the highest-ranked American man. "If he's sweating a lot, and has to change clothes, then you know it's pretty humid out there."

Isner estimated that he might have lost 8 to 10 pounds during his quarterfinal loss Tuesday in Ashe to 2009 champion Juan Martin del Potro, who'll meet Nadal in the semifinals.

"Whatever the humidity is on outer courts or in the city," Isner said, "I think it's amplified on center court. It's just we have seen it all tournament long — the girls and the guys out there, just sweating a lot."

One player who wasn't sure what the fuss was all about was 20th-seeded Naomi Osaka, who represents Japan and trains in Florida.

"I actually don't think it's that hot. Sorry," Osaka said after her quarterfinal victory. "I don't know, I think it's because I'm used to the Florida heat. So yeah, I kind of enjoyed it. I like sweating."

It's been so unusually muggy that the U.S. Tennis Association, which runs the Open, made up a rule on the fly to offer some help to the men, letting them take a 10-minute break between the third and fourth sets of a match that goes that long if it's too hot.

There already was a rule on the women's tour offering that sort of pause between the second and third sets.

When junior and wheelchair matches were suspended Thursday, it meant things got bad enough to warrant some sort of concession to the heat on seven of the tournament's 11 days.

Some, including Djokovic, have wondered whether the retractable roof on Ashe could be closed to allow the air-circulation system to reduce humidity. But tournament policy is that the stadium can only be covered when it's raining or showers are predicted, as was the case ahead of the women's semifinals Thursday night.

"Nobody remembers players dealing with these type of heat and humidity conditions for this extended period of time. We understand the conditions are very tough inside of Arthur Ashe Stadium. Players are talking about. We hear them," USTA spokesman Chris Widmaier said.

"It's an open-ended question: What can be done now?" he said. "There's no magical solution to this."

He said that the four doors in the 23,771-seat main arena's corners are being propped open during matches, but the effect has been negligible.

Federer said he had a hard time breathing. The man who beat Federer, 55th-ranked John Millman, said it was hard to hold his racket. It was Millman, in his loss to Djokovic, who received permission for the unusual mid-set outfit swap because he was leaving sweat all over the court.

"Whether it's night or day, we just don't have air down there," Djokovic said. "It feels like sauna."

Given all of the sweating going on, he was asked whether the players' changing areas are smellier than usual this year.

"What happens in locker room," Djokovic replied with a smile, "stays in locker room."

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