How the top players have beaten the heat at this year's US Open

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Novak takes shirt off during match break with Millman (1:06)

John Millman requests time to change his clothes and Novak Djokovic sits shirtless in his chair. (1:06)

NEW YORK -- Up a set and 2-2 in his quarterfinal match Wednesday night, Novak Djokovic sat shirtless in his changeover chair for nearly six minutes as he waited for his opponent, John Millman, to return from the locker room wearing a dry change of clothes. Three games later, Djokovic returned to the chair to change into a fresh pair of socks and shoes. That's but a small taste of the bizarre scenes created over the past two weeks at the US Open by this stifling New York heat.

From the number of retirements in the early rounds (13 in total, third-most at any Slam in the Open era), when temperatures soared near 100 degrees, to a flurry of players restringing their rackets mid-match to account for the rising heat, the final Grand Slam of the season has been an extreme affair. But if our sometimes-trusty weather apps are to be believed, cooler days lie ahead for the players who managed to survive into the semifinals. So how'd they manage to do so? Read on ...

THEIR MATCHES (AND RACKETS) WERE TENSION-FILLED

Last Wednesday, on the first extremely hot day of the tournament, the Wilson racket-stringers were some of the busiest folks on the US Open grounds. "Typically, when it's hot, the players will go up in [string] tension," said Dustin Tankersley, captain of the Wilson stringing team at the Open. "That day, we did 26 'on-courts,' which is when a player gives a racket to a ball kid during a match, they bring it to us in the main stringing room, we cut the strings, restring and stencil it and send it back out to the court with the ball kid."

When the temperature is hot, the ball flies through the air more quickly, so players lose some control of the ball off their rackets. The high heat also causes the strings to lose tension more quickly. To counteract both problems, Tankersley said the majority of players increase their racket tension by 2 pounds. Millman, for example, started the tournament with his rackets strung at 62 pounds, but increased to 64 for his night match against Roger Federer, and then used that same tension Wednesday night against Djokovic.

For some, 2 pounds isn't even enough. "When I coached Andre [Agassi], on a day like today, he might string his racket 5 or 6 pounds tighter because the court's playing quicker," ESPN analyst Brad Gilbert said. "At night, when it's more humid, maybe he would loosen it up."

And then there are players who don't allow the weather, no matter how extreme, to dictate their equipment. "Some players change their string tension for grass, clay, hard courts, as well as the heat," Tankersley said. "But [Rafael] Nadal doesn't change his tension ever. It's a factor he doesn't think he needs to worry about."

But it's not just the strings players are meticulous about in this heat; choosing the right grip material is crucial as well. Some players regripped their rackets using a more absorbent material to help soak up sweat -- and then layered on wristbands for added absorbency. "The conditions have been pretty brutal," Millman said. "I found it tough early on to hold on to the racket."

After beating Sloane Stephens in a plus-90-degree noon match Tuesday, Anastasija Sevastova said she, too, struggled to keep her racket handle dry. "For the hands, [the humidity] is not nice," she said. "Your hands are always slippery. I don't like it, but it is what it is. It's the weather in New York right now."

IT'S NOT THE HEAT -- IT'S THE HUMIDITY

That's not just a phrase your grandparents utter when you visit them at their winter home in Florida. It's true. Extreme heat can wreak havoc on a player, but high humidity is far more dangerous, robbing the body of its ability to cool itself down.

"Sweating is your body's way of releasing heat," said Dr. Melissa Leber, a player physician for the US Open and director of emergency department sports medicine at The Mount Sinai Hospital in NYC. "But sweat doesn't evaporate as well in the humid air. That makes it difficult to cool down. Your body doesn't respond appropriately and instead says, 'I'm overheating. Let me produce more sweat and more sweat.'"

That's why, after his quarterfinal match against Juan Martin del Potro on Tuesday afternoon, John Isner said he changed sweat-drenched T-shirts 11 times, as well as his socks and ankle braces, which became waterlogged and heavy. That's one way to help to dissipate sweat and assist the body in its cooling-off process.

At 6-foot-10 and 238 pounds, Isner said he believes he lost between 8 and 10 pounds during the match, which is a dangerously high percentage of body weight to lose in such a short amount of time. "I have always said it's pretty difficult to play in hot conditions, for me especially, because I weigh a lot and I sweat a lot," Isner said. "It takes its toll on bigger guys."

Said Leber: "There may be a genetic component, a male vs. female component and also a height issue to how players take the heat differently. Some people lose more sodium than others, and some have more water in their sweat. A lot of the top players have their sweat rate and electrolytes measured and work with nutritionists to create a hydration plan that starts in the days leading up to a match."

On court, fluids, ice towels and breaks are used to help players prevent heat illness, and once the symptoms set in -- headaches, cramps, disorientation -- medics like Leber are armed with old-fashioned salt packets to help the players rehydrate. "Once they're symptomatic, there's not much we can do in-match except encourage and support them with ice towels," she said. Prevention, then, is key.

"SLOW COURTS" = SLOW BALLS

Madison Keys, who is into the semifinals in New York for the second consecutive year, said that during her quarterfinal match Wednesday night, "the court's playing slower." She then went on to explain what physicists already know. What's "slow" isn't necessarily the court itself, but the ball's reaction off the playing surface. "Because it's been so humid," she said, "the balls are getting a lot fluffier and playing even slower."

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Keys cruises past Suarez Navarro to US Open semifinals

Madison Keys and Carla Suarez Navarro get into a rally and Suarez Navarro's return goes long as Keys wins the match in straight sets 6-4, 6-3.

Keys might be on to something. High temperatures increase the ball's speed in the air and the gas inside the ball expands, resulting in a higher bounce, which benefits players with heavy topspin. (Nadal, anyone?) But when it's humid, as it was Wednesday, the ball absorbs more water and becomes heavier and "fluffier," a common word used by players to describe a slow, wet ball.

Keys said she believes it's no coincidence that three of the four finalists in the women's draw -- Keys, Serena Williams and Naomi Osaka -- all train in Florida and know how to adjust for the humid conditions.

COOLER HEADS PREVAILED

After losing in stunning fashion to Millman on Monday night, Federer said he felt "everything was off" for him as he struggled to breathe at center court, and that Millman, who trains in the hot, humid climates of Brisbane, Australia, "was able to deal with it better." That's a phrase several players have used to describe their triumphant opponents. But dealing with the humidity isn't limited to who has the better hydration plan.

"There is an unbelievable mental side to this equation," Gilbert said. "Some people are obviously a little fitter and can actually handle extreme conditions better than others. But if you have an opponent that's making you work harder, everything can change within the match. When the heat was oppressive, Agassi would get excited. It's not like he relished going out in a million degrees. He just knew that if he went phenomenally hard for 47 minutes, he could break somebody's will and desire to compete."

Arguably, that's what Osaka did Wednesday afternoon. Playing in the scorching heat of the midday sun, she defeated Lesia Tsurenko 6-1, 6-1 in 57 minutes. After the match, as player after player decried the impossible conditions, Osaka said she barely noticed the heat. "I actually don't think it's that hot," she said. "I'm sorry. I'm used to the Florida heat. I kind of enjoyed it. I like sweating."

With that attitude, Osaka might beat more than the heat over the next few days.