Serena Williams out of Australian Open
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.
Stephens won the second set when Williams, serving slower than usual, was broken. Williams also had her service broken in the final game of the match.
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.
Copyright 2013 by The Associated Press
This story is from ESPN.com's automated news wire. Wire index