Thirty minutes before the match against her American opponent, the defending Australian Open champion announced she would be unable to compete after undergoing a minor procedure to fix an ingrown nail that infected her right big toe.
"It's been there for about 10 days," the 23-year-old Belarussian told reporters.
"It's just been getting worse a little bit," she added.
"I don't know in medical terms, but I had to get a piece (of nail) out of my toe because it was jamming into me and got infected.
"It's something that's very just really unfortunate, but I had to do that. It was just jamming underneath into the skin. I was trying to minimize the pain with taping and everything.
"Yesterday it got worse. It got really infected and got really red, so we had to go and see the doctor. He had to open it. That's what I had to do."
Azarenka had won her first two matches at Brisbane without any signs of discomfort but the eagerly-anticipated showdown against the third-ranked Williams failed to materialize.
"It just got infected from a bad pedicure experience," she said.
"I had somebody who gave me infection. They cut a little bit too much and it got infected. And then from as much as we do, as much exercise, it's just been jamming into it and it's created an infection inside. Created a little piece of nail to go in and always be hurting me.
"Actually the doctor said it happens a lot. Never happened to me, so it's something that I am definitely going to prevent for the next time.
"I tried everything. We tried medication with taping, and I was playing through the pain for quite a while.
"You know, it's just something that I had to do to make sure that I can be fully recovered and ready for Australian Open. It's the compromise I had to take.
"It's just very unfortunate timing, because I was really looking forward to playing. But the health is definitely something that's more important."
Despite the setback, Azarenka expected to be fully fit when the Australian Open begins in Melbourne on January 14.
"The procedure has been done and the worst already passed by. It just needs some time to get it better."
Williams will now meet either Russia's Anastasia Pavlyuchenkova or Ukraine's Lesia Tsurenko in the Brisbane final on Saturday.
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