Williams appeared to hurt her lower back while attempting to pull up short of the net during the eighth game of the second set. She called for a trainer after the ninth game with the score 5-4 and on serve.
At 2-1 and on the way to a changeover in the third set, Williams smashed her racket angrily on the court then threw it toward her chair.
Stephens will play defending champion Victoria Azarenka in the semifinals.
By Elaine Donnelly
Extending sexual misconduct to combat units
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