Azarenka made the announcement Friday about a half hour before she was due to play No. 3 Williams, who is on a roll after winning titles at Wimbledon, the London Olympics and the U.S. Open.

The withdrawal came shortly after Andy Murray, the Olympic men's gold medalist and U.S. Open champion, finished off his 6-4, 7-6 (3) win over Uzbekistan's Denis Istomin in the quarterfinals.

Murray had lapses in his opening three-set win over No. 199-ranked Australian qualifier John Millman on Thursday night, while Ivan Lendl was en route to Australia, but had no serious trouble with his coach in the stands.

The first of Murray's three titles in a breakthrough 2012 season was in Brisbane, where he started working with former No. 1-ranked Lendl last January. The next two titles were career-defining triumphs at the London Olympics and U.S. Open.

Murray said he was satisfied with his serve _ he didn't get broken in the match _ and was starting to get over the aches and pains that are usual when returning from the end-of-season break.

``When you're practicing it's hard to replicate match situations ... and they're the things that take a while to get back into the rhythm of playing,'' he said, putting his relatively slow start in Brisbane down to a lack of match practice rather than a lack of condition and rejecting the suggestion he had been ``rusty.''

He next plays fifth-seeded Kei Nishikori of Japan, a 6-4, 7-6 (3) winner over 2012 Brisbane finalist Alexandr Dolgopolov.

The other semifinal will feature 2006 Australian Open finalist Marcos Baghdatis, who ousted No. 3 Gilles Simon of France 6-3, 6-4, against 21-year-old Bulgarian Grigor Dimitrov, who beat seventh-seeded Austrian Jurgen Melzer 6-3, 6-2.

Murray didn't face a breakpoint in the second set, and only lost one point in his first three service games of the set _ a call he disputed but lost on the challenge review.

He held serve to force a tiebreaker and trailed Istomin 3-1 before winning six straight points. Istomin served the only double-fault of the match to surrender triple match point and Murray hit a forehand winner to clinch it in little more than 1 1/2 hours.

Murray dropped a set to Millman and appeared to be sore and fatigued at times on Thursday night but there was no such issues in front of Lendl, who has played a key role in ending the Scottish player's run of four defeats in Grand Slam finals before he finally won a major. Lendl also lost four major finals before his claiming the first of his eight Grand Slam crowns.

With her impressive finish to the 2012 season taking her major tally to 15, Williams is the most formidable player in women's tennis.

She has won 34 of her last 35 matches since a first-round exit at the French Open and is in contention to return to the No. 1 ranking if she wins the Australian Open.

Williams also has won 11 of her 12 matches against Azarenka, including all five in 2012.

After missing the chance for a rematch, she advanced directly to a final against the winner of Friday's semifinal between Russia's Anastasia Pavlyuchenkova and Ukraine's Lesia Tsurenko.

Azarenka said she'd picked up an infection after a pedicure last week and had to have part of the nail on the big toe of her right foot removed. She was planning to head to Melbourne and hoping it would only take two days before she can continue fully preparing for the season's first major.

``I tried everything. We tried medication with taping, and I was playing through the pain for quite a while,'' said Azarenka, who won the Sydney International and the Australian Open last year during a 26-match winning streak to start the season. ``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 that sort kind of compromise I had to take.

``It's just very unfortunate timing, because I was really looking forward to playing and excited.''

Williams said she could empathize with Azarenka.