Victoria Azarenka collapsed suddenly during her match against Gisela Dulko at the U.S. Open on Wednesday.
Azarenka had been bending her knees and leaning on her racquet between points. She was walking behind the baseline preparing to serve down 1-5, 15-30 when she appeared to lose consciousness and fell on the ground. She was treated by medical staff on court and then taken off in a wheelchair.
A statement released later by the tournament said Azarenka retired with "headache-like symptoms" and "was taken to a nearby hospital for diagnostic testing.
No details were given about her condition but the statement said the cause "does not seem to be primarily a heat-related illness."