US Open, Flushing Meadows
Dates: 30 August - 12 September Start time: 1600 BST
Coverage: Live text commentary on BBC Sport website and mobiles; updates on BBC Radio 5 live; live on Sky Sports
Azarenka called the trainer in the third game in soaring temperatures
Tenth seed Victoria Azarenka was taken to hospital after collapsing on court during her second-round match with Gisela Dulko at the US Open.
As temperatures soared in New York, the 21-year-old Belarussian slumped to the ground while serving at 1-5 and 15-30.
She left the court in a wheelchair, and later revealed the problem was the result of an earlier fall in training.
"I was taken to the hospital for some medical tests and have been diagnosed with a mild concussion," she said.
Azarenka appeared to struggle as early as the third game in on-court temperatures that touched the 100-degree mark, but while the heat cannot have helped, she later explained the circumstances behind her collapse.
I was scared. She went to the floor - I was worried for her
"I was warming up in the gym prior to my match 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."
In an earlier statement, US Open tournament referee Brian Earley had said: "Victoria Azarenka retired from her match with headache-like symptoms. She was taken to a nearby hospital for diagnostic testing.
"Out of respect for her privacy, we cannot give any more details. However, we can say that this does not seem to be primarily a heat-related illness."
Dulko added: "I was scared. She went to the floor - I was worried for her. I went to see her, brought her some ice, did whatever I could to help.
"I'm used to this kind of weather but on the first day when I went to practice I was feeling dizzy as well. It's hard to play in these conditions."