Women's top seed Victoria Azarenka was forced to pull out of her semi-final of the Brisbane International due to a toe injury, handing Serena Williams an automatic place in the final.

Azarenka, from Belarus, reluctantly dropped out in order to be fit for the defence of her Australian Open title.

Thirty minutes before the match against her American opponent, Azarenka 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 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 minimise 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."

Despite the setback, Azarenka is expected to be fully fit when the Australian Open begins in Melbourne on 14 January.