After having her left flank massaged for 33 minutes, the player decided to throw in the towel.
He is now in his third retirement after his senior debut last year, when he was just 18 years old at the Nottingham tournament. She was already forced to leave the court in Guadalajara, February, and Rome last month.
She will be playing next week in Birmingham, then returning to Wimbledon. Last year she was a big draw with her appearance in the round of 16, and then became a star when she won the US Open in September.
The world No.11 is still in difficulty after her coronation at Flushing, Meadows. She was eliminated in the second round Roland-Garros in Paris by Belarusian Aliaksandra Ssnovich in three sets.
