"Unfortunately Bogdan turned her ankle and couldn't put any weight on it. I didn't see it happen, but if she had got through two more points she would have come off for the heat, so it must have been bad."
I'll just add this here as well, more detail from the BBC:
"Bogdan was leading 5-1 in the second set and had deuce when she turned badly on her left ankle. She limped back to her chair and had treatment from the doctor and physios.
"She wanted to continue because she knew if the match went to one set all, the extreme heat policy would be enforced the players would be brought off.
"So she limped back to the baseline and played four more points (actually winning two of them) before the doctor walked onto the court and basically forced her to retire.
"Bogdan was in tears and her coach was shaking his head but she was in no position to play and for her own good it was the right decision. Referee Scott Ray has just confirmed that the Doctor has the power to take such a decision if it's in the player's best interests."