No.10 seed Victoria Azarenka was taken to hospital after collapsing on court in dramatic scenes at the US Open on Wednesday.
Azarenka was trailing Argentine Gisela Dulko 5-1 in the first set of her second-round match in hot, humid conditions when she fell to the ground.
Medical personnel rushed to her aid and yelled to the sideline for water, before helping the Belurusian into a wheelchair and taking her away for further medical attention.
Azarenka seemed to be battling the heat in the early part of the match before calling for the trainer in the third game and complaining of an arm injury and blurred vision.
As the match wore on, the 21-year-old became increasingly ill-at-ease, taking time out between every point and looking wobbly on her feet.
Then, while serving in the seventh game at 15-30, Azarenka collapsed on the baseline amid gasps from the crowd.
She later revealed she had injured herself earlier in the day.
"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."
"I was taken to the hospital for some medical tests and have been diagnosed with a mild concussion."
The Belarusian's retirement from the match ensured an easy passage into the third round for Dulko, who will now meet Anastasia Pavlyuchenkova.
Following the match, Dulko said: "After that you don't think about tennis, you are just worried about your opponent."
"Now she has gone to the hospital so I guess it's something serious and I hope she is feeling good soon."
Speaking of the conditions, she said: "It's hard to play in these conditions and today I am much better because I have been here a few days, but it's tough."