Sweat flew as Rafael Nadal returned a shot during his match against David Ferrer in Arthur Ashe Stadium. (RAY STUBBLEBINE/EPA-EFE/Shutterstock)

As John Millman duckwalked off the court in the middle of his match to change his sopping clothes Wednesday night at Arthur Ashe Stadium, Novak Djokovic peeled off his own soaked shirt, laced his hands behind his head and leaned back in his courtside chair with a smile.

“I’m fine to have a little rest,” Djokovic said cheekily as the crowd roared in amusement. Millman was so sweaty that he had gotten special permission to change at 2-2 in the second set of their U.S. Open quarterfinal. Breaks like those usually happen only at the end of a set, but Millman was actually creating a hazard — his sweat was pooling on the court. He or Djokovic could have slipped.

The U.S. Tennis Association promptly released a statement clarifying what had happened, the governing body’s 12th statement in the first 10 days of action in the U.S. Open’s main draw. Eight of those statements have been about the heat, three have been about unruly chair umpires acting out of turn — and one was about an Australian player’s excessive perspiration.

It has been a weird U.S. Open. Weird — and sweaty.

“I personally have never sweat as much as I have here,” Djokovic said early Thursday morning after completing his 6-3, 6-4, 6-4 victory.

“You don’t stop sweating, though,” Millman added. “You go to this little holding room just off the court, and there’s a tiny, probably like, 3-by-3 room, and you’re just dripping. The sweating doesn’t stop.”


Novak Djokovic revels in a break that came when quarterfinal opponent John Millman had to leave the court to change his sweat-soaked clothing. (EDUARDO MUNOZ ALVAREZ/Agence France-Presse/Getty Images)

It has been uncomfortably hot at Billie Jean King National Tennis Center since the start of the U.S. Open, with temperatures in the 90s and humidity around 50 percent that has regularly raised the heat index above 100 degrees. On six of the tournament’s first 11 days, the USTA invoked an extreme heat rule that affords players an extra 10-minute, mid-match break.

The harsh sun is unforgiving on the tennis center’s roofless, outer courts, but heat isn’t really the problem inside Arthur Ashe Stadium. Even when open, the roof provides shade for all but the earliest match of the day. Humidity is the real issue. Near the court where the players, line judges, ball kids and umpires scurry about, the air is still and thick.

“I don’t think that’s a normal occurrence,” John Isner said Tuesday after his quarterfinal loss to Juan Martin del Potro. “It’s been pretty hot here, and it can be — I don’t think, whatever the humidity is on the outer courts or in the city, I think it’s amplified on center court.”

It even affected the most elegant player in the men’s game.

Conjure an image of Roger Federer in action — his hair flopping perfectly, his feet seemingly never touching the ground. Yet the second-seeded Federer perspired so much during his upset loss to Millman this week that some began to wonder whether his switch in apparel sponsorship from Nike to Uniqlo was to blame.

“You know, I have never seen Roger sweat, ever,” Isner said. “If he’s sweating a lot and has to change clothes, then you know it’s pretty humid out there.”

“I guess I felt I couldn’t get air,” Federer said with a shrug after his fourth-round loss. “There was no circulation at all.”

Djokovic, who has suffered through his share of hot daytime matches in this tournament, felt so stifled that he spoke to the chair umpire about it Wednesday.

“I asked the chair umpire whether they are using some form of ventilation or air conditioning down at the court-level side, and then he says that he’s not aware of it — that, you know, only what comes through the hallway type of thing,” Djokovic said. “I think that this tournament needs to address this. I mean, because whether it’s night or day, we just don’t have air down there. It feels like a sauna.”

A handful of players, including Federer and Djokovic, have posited that Arthur Ashe Stadium’s retractable roof, which is attached to a 5,000-ton steel superstructure built around the stadium, is trapping humidity on the court. Since the U.S. Open is an outdoor tournament, the USTA keeps the roof open as much as possible and closes it only in the event of rain. The roof was closed Thursday afternoon, before the women’s semifinals in the evening, in anticipation of thunderstorms.

“I do believe, since the roof is on, that there is no air circulation in the stadium. I think just that makes it a totally different U.S. Open,” Federer said. “Plus conditions maybe were playing slower this year on top of it. You have soaking wet pants, soaking wet everything. The balls are in there, too. You try to play. Everything gets slower as you try to hit winners.”

But even when the roof is open, there isn’t much airflow. The stadium has an air conditioning system of sorts — a water-cooled ventilation system blows air into the seating areas — but it’s difficult to effectively cool the cavernous, nearly-24,000-seat facility.

Talk of the humidity has come mostly from men’s players, who play best-of-five sets at Grand Slams, though women with afternoon matches have spoken up as well.

For the men, the humidity means packing 10 or 11 shirts to keep up with the sweat as well as changing shoes on court, as Dominic Thiem did in his epic, 4-hour 49-minute quarterfinal loss to Rafael Nadal. It’s hard to run in soaking wet shoes.

It gets harder to do anything in humidity like that.

“I can only talk about myself,” Djokovic said. “This has been definitely the toughest U.S. Open in the last, you know, 10 years that I have played in, in terms of conditions. Yeah.”