NEW YORK -- Victoria Azarenka collapsed and had to be taken off the court in a wheelchair Wednesday, an accident that resulted from a concussion she endured after falling and hitting her head while warming up for her U.S. Open match.

The 10th-seeded Belarusian was trailing Gisela Dulko 5-1, 31 minutes into her match on another steamy day at Flushing Meadows, when she stopped running and crumpled to the ground. She was taken to the hospital for tests, diagnosed with a mild concussion. Victoria Azarenka was transported to a hospital after being treated at the U.S. Open. AP Photo/Paul J. Bereswill

A scary scene on a day in which the temperature had already reached 90 degrees when the accident occurred at 11:30 a.m. However, the 21-year-old released a statement saying it was a head injury, not the heat, that set her up for the fall.

"I was warming up in the gym prior to my match against Gisela Dulko when I fell while running a sprint," she said. "I fell forward and hit my arm and head. I was checked by the medical team before I went on court and they were courtside for monitoring. I felt worse as the match went on, having a headache and feeling dizzy. I also started having trouble seeing and felt weak before I fell."

Wearing an all-black tennis dress, Azarenka had been moving slowly throughout the match. Before she served the second point in the seventh game, she hesitated and winced in pain. After the point, she kneeled down.

A point later, while trying to run on the baseline, she stopped and crumpled to the ground. Medical personnel rushed out, draped a towel across her legs and yelled to the sideline to bring water. After a few minutes on the ground, being sheltered by an umbrella and with an ice pack on her neck, Azarenka was helped into the wheelchair and moved off the court. A trainer placed 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. For the second straight day, tournament officials put in place their extreme-weather policy, meaning women could ask for a 10-minute break if they split sets.

Temperatures were in the mid-90s at midday.

This wasn't the first time Azarenka has dropped out suddenly from a Grand Slam tournament.

At the 2009 Australian Open, she won the first set of her fourth-round match against Serena Williams but quit during the second set while dizzy and in tears during a hot day in Melbourne.

Though the Aussie Open -- played in midsummer in the Southern hemisphere -- may be the most consistently hot Grand Slam tournament, they have roofs available on some of their courts. That makes the U.S. Open, where all matches are played outdoors, the toughest grind of the four major tournaments in many minds.

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.

The news of Azarenka's withdrawal traveled 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. It took more than five hours for Azarenka and the tournament to confirm that heat was not the reason for the accident.

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 Williams, the eventual champion.

Edmonton Oilers goalie Nikolai Khabibulin, a native of Russia, is a family friend and a mentor, taking her in with his family in Scottsdale, Ariz.