Azarenka, who last year won the Sydney International before claiming her first Grand Slam title at the Australian Open during a 26-match winning streak, said Friday she was advised not to play after receiving treatment for an infection on the big toe of her right foot.
She has lost all but one of her previous 12 matches against Williams, a 15-time major winner, and was 0-5 in head-to-heads in 2012 including a defeat in the U.S. Open final.
Williams will meet the winner of the other semifinal between Russia's Anastasia Pavlyuchenkova and Ukraine's Lesia Tsurenko in Saturday's final.
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