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.
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