Halep withdraws from Bad Homburg, raising doubts over Wimbledon
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The Romanian, 29, said in a video message on her Twitter account that "my calf is not ready for a competition" at the grasscourt event in Germany.
She was also ruled out of the French Open with the same problem.
The world number three sustained the injury in her second-round match at the Rome Open on May 12.
This year's tournament begins on June 28.