Heat-related illnesses have taken a handful of players out of the US Open, and while so far those issues were not serious, dehydration and other conditions could prove dangerous as the final tennis Grand Slam of the year rolls on.
With temperatures in New York soaring into the 90s Fahrenheit (32 degrees Celsius), accompanied by punishing humidity, tennis players are feeling the heat -- the air feels heavier and it's harder to breathe.
Five players have retired so far because of the conditions, and many others have complained about the sweltering temperatures.
Novak Djokovic summed it up in two words: "Brutal conditions."
Robert Glatter, an emergency room physician at Lenox Hill Hospital in New York and a former sideline doctor for the NFL's New York Jets, explained that lack of water can lead to serious performance issues.
"They're not having heat strokes -- that's usually in older people, people with heart and kidney disease, where they stop sweating," Glatter told AFP.
"What they do have is the early signs of dehydration, and maybe heat cramping, which is more likely related to inadequate hydration and nutrition."
Heat stroke is the most serious heat-related illness. The body temperature can rise to 104 Fahrenheit (40 Celsius) or beyond. A sufferer's face turns bright red, and eventually, he or she can fall into a coma.
Other tell-tale signs for concerns among athletes competing in extreme heat are unusual fatigue or vertigo.
Glatter says the key is to remain hydrated. Even elite athletes can fail to drink enough water, he says.
"What we're experiencing here in New York is beyond what they're typically trained for," he said.
On Tuesday, US Open organizers put into effect their Extreme Heat Policy -- women are given a "heat break" between the second and third sets, and men are offered a similar 10-minute off-court rest prior to a fourth set.
Those breaks "allow the heart rate to slow down, and allow adequate fluid intake," Glatter said, calling them "very beneficial."