The 10th-seeded Belarusian was trailing Gisela Dulko 5-1 in the first set on the Grandstand court, where the temperature had reached 32degC with 42 percent humidity by 11:30am.
Tournament officials gave no immediate update on her condition. About an hour after the match was called, her agent said Azarenka was being transported to the hospital. Dulko said Azarenka was conscious and responding when she walked over to make sure everything was OK.
Wearing an all-black tennis dress, Azarenka had been moving slowly throughout the match and didn't appear to twist or sprain anything when she fell to the ground.
Medical personnel rushed out and yelled to the sideline to bring water. After a few minutes on the ground, being sheltered by an umbrella, Azarenka was helped into the wheelchair and moved off the court, with a trainer placing his hand on her neck to check her pulse.
"I was scared," said Dulko, who advanced to the third round. "She went to the floor. I was worried for her. I went to see her, brought some ice, did whatever I could do to help."
Dulko said the conditions were, indeed, brutal, though as of 12:30 p.m., tournament officials had not put in place their extreme weather policy, the way they did on Tuesday, when temperatures reached into the mid-30s. It was expected to be as hot today.
"It's tough to play out there," Dulko said. "It's really hot, really humid. You sweat so much, sometimes it's impossible to hold the racquet."
Though the Australian Open - played in mid-summer in the Southern hemisphere - may be the most consistently hot Grand Slam tournament (with a roof available on the stadium court), the US Open has a well-earned reputation of being the toughest grind of the four.
The year 2010 is going down as the hottest summer in recorded history in New York, which certainly is doing nothing to change that reputation.
Seven men's matches went five sets and 14 women's matches went three sets in the stifling heat on Tuesday, though Azarenka was the first to fall with what appears to be a case of heat exhaustion.
The news travelled quickly through the tennis world.
"Did anyone watch Vikas match?? I really hope she is ok!" top-seeded Caroline Wozniacki said on her Twitter account, a few minutes after Azarenka fell. The previous night, Wozniacki defeated Chelsey Gullickson 6-1, 6-1 in a match that ended around 1 a.m.
The 21-year-old Azarenka has won four career titles, most recently at Stanford last month, when she beat Maria Sharapova in the final. She reached the quarterfinals at the Australian Open this year, losing to eventual champion Serena Williams.
