Andy Murray prepared for the steamy conditions that are common at the US Open by the “brutal heat and humidity” of New York this time of year with the help of a steam room at his home.

The 36-year-old British tennis star, who lost in the second round, set the humidity in there at 70% and spent hours riding a stationary bike nearby with the thermostat cranked up to 35 degrees Celsius, making the air feel as muggy as it does every summer around Flushing Meadows, where the year’s last Grand Slam tournament entered its second week on Monday. 

Temperatures at the 2023 US Open have not been as high as usual this year, giving athletes, ball crews and spectators some reprieve. “It is a little cooler than usual; that’s definitely easier to play in,” Belgian player Elise Mertens said last week. From Sunday though, the temperature has risen to 32 degrees Celsius, and is forecast to rise further. 

That’s not a surprise: An AP analysis shows the average high temperatures felt during the US Open and the three other major tennis tournaments steadily have gotten higher and more dangerous in recent decades, reflecting the climate change that caused heat waves around the world. For athletes, it can keep them from playing their best and increases the likelihood of heat-related illness.

AP tracked the thermal comfort index, which measures air temperature in degrees while also taking into account humidity, radiation, wind and other factors that affect how the body responds. It looked at each Grand Slam event dating to 1988, the first year all four had 128-player fields for women and men. Collectively, the maximum temperatures at those tournaments has risen by nearly 3 degrees Celsius.

“People hear that and they don’t think it’s very much. It doesn’t necessarily register as alarming. Sometimes that 3- or 4-degree change can cause a doubling or even tripling of the number of hot days we experience,” said Daniel Bader, a climate scientist at Columbia University. “New York City’s temperatures have been rising, and that trend is projected to continue into the future.”

From 1988 to 1992, daily highs in the thermal comfort index passed the threshold for strong heat stress, which is 32 degrees Celsius, on 7% of days with Grand Slam matches. From 2018 to 2022 that figure was 16%.

The US Open’s overall rise of nearly 1.5 degrees Celsius since 1988 means it isn’t even the Grand Slam site where the heat is increasing most rapidly. That’s the Australian Open, where the average high temperatures jumped by more than 3.5 degrees Celsius.

The U.S. Open’s spot late in the tennis season creates an accumulation of wear-and-tear and general fatigue, but the sweltering conditions at Flushing Meadows likely deserve some blame for a high number of in-match retirements there. “I remember the year I won, the last four days it was super hot and super humid,” said 2016 U.S. Open champion Stan Wawrinka of Switzerland. “It’s one of the toughest tournaments, fitness-wise. … Your body really loses a lot of energy.”

Since 1988, there have been 17 occasions in which at least 10 players at one Slam stopped during matches, more than half of them at the US Open. “We’re seeing a lot more heat-related illnesses across all sports,” said Elan Goldwaser, a sports medicine physician at Columbia University Medical Center who works with athletes on the US ski team and at Fordham University.

The blue hard courts at the US Open absorb heat more than the grass at Wimbledon or the clay at the French Open, making it feel as much as 8 degrees Celsius hotter than the air temperature, according to the US Tennis Association. Athletes “are essentially playing on a hot plate,” Goldwaser said.

“Their ability to hit the ball as hard starts to go down. Their reaction time starts to go down,” said Jon Femling, the clinical vice chair of emergency medicine at the University of New Mexico. “Getting heated up, your body’s first response is to try and cool down, and the way it does that is by pumping blood to all of your skin. … Your heart just has to immediately start working harder.”

At the US Open, players get 75 seconds to rest between games and two minutes between sets. That’s time enough to hydrate with water or electrolyte-packed drinks, enjoy cold air pushed through a tube or wrap an ice-filled towel around their neck. It’s not enough time, though, to lower the body’s core temperature. So physiotherapists watch for dizziness, cramping and other signs of heat illness.

“For a recreational player who may be watching on TV, to think about playing a tennis match when it’s 35 degrees Celsius and 95% humidity — they might think that’s just unfathomable,” said Todd Ellenbecker, ATP vice president for medical services. “But our players … play in that kind of heat throughout the year.”