Can anyone stop Serena in final four?

She clinched the year-end No. 1 ranking, but Victoria Azarenka wasn't sharp in dispatching Li Na 7-6 (4), 6-3 to reach the semifinals at the end-of-season championships in Istanbul. Up next for Vika is Maria Sharapova in a rematch of the Australian Open final.

Serena Williams, meanwhile, benefiting from a day off, tangles with Agnieszka Radwanska in a repeat of the finale at Wimbledon after the Pole outlasted Sara Errani 6-7 (6), 7-5, 6-4.

Here's a peek at Saturday's semis:

Victoria Azarenka (2-1) versus Maria Sharapova (3-0)

Sharapova showed Azarenka how to do it Friday. Already guaranteed first place in the White Group, Sharapova easily could have packed it in against Samantha Stosur, as Azarenka did last year at the year-end championships in a meaningless encounter (for her) against Marion Bartoli.

It was never going to happen. This is Sharapova, one of the game's finest competitors and a real pro. She spent an hour on court in disposing of an out-of-sorts Stosur, who put together one of the worst sets of tennis this year (five points won in the first).

What is it with Aussies lately and ugly sets?

Although the respect is there, Azarenka and Sharapova don't like each other. There was that bump in Stuttgart, Germany, this year, when Sharapova became annoyed as Azarenka took an injury timeout, and Azarenka's celebratory dance following her victory over the Russian at the U.S. Open had to be noted by her opponent. Maria doesn't like those dancers, right, Andrea Petkovic?

Sharapova is 1-4 against the world No. 1 in 2012. All four losses came on hard courts, and in only one of them, in New York, did Sharapova snare a set. In Beijing this month, Azarenka gave up four games and Sharapova delivered seven double faults. A combination of feeling the heat and trying to inject more into the second serve spiked the doubles.

But Azarenka hasn't been herself this week. She labored for three hours against Angelique Kerber and failed to protect a 3-0, double-break, second-set advantage against Williams. Her racket went flying in both tussles.

It was a continuation against Li in Friday's must-win match. The Chinese baseliner was a step quicker for most of the first set. Azarenka made strange challenges, struck successive double faults and was fortunate that Li coughed up a pair of doubles herself trying to serve out the opener. Li closed the gap from 5-1 to 5-3 in the second.

Since the draw was made, the complexion has changed.

Prediction: Sharapova in two

Serena Williams (3-0) versus Agnieszka Radwanska (2-1)

After losing a 72-minute first set to Errani in the White Group shootout for second place, Radwanska probably was thinking, "Do I really want to play three hours just so I can get Serena tomorrow?"

Radwanska, unsurprisingly, waned early in the second set, and checking her pulse perhaps indicated she wasn't feeling great. Still, she displayed her resilience once again to set up Saturday's clash with the 15-time Grand Slam champion. In the end, she went more than three hours: 3 hours, 29 minutes.

Radwanska eventually outfoxed Errani in a thriller in which both hit more winners than unforced errors. Radwanska's tally of 53 winners must be a career high.

Spare a thought for Errani, who now will turn her attention to doubles. Yes, in case you weren't aware, the top four doubles teams also are in Istanbul. Errani relinquished a 2-0 lead in the second set, a time when Radwanska seemed vulnerable.

Radwanska, in her postmatch interview, was asked about meeting Williams.

"First of all I will try to [have] treatment to stay in one piece," said Radwanska, a loser to Sharapova in 3:12 in a match that ended at about 2 a.m. in Istanbul on Thursday. "I have nothing to lose. I've had a lot of tennis in the last three days."

Williams needed three sets to cast aside the pesky Radwanska at Wimbledon, although it was a swoon by the former in the second set that extended proceedings. Eliminate that set, and Williams has won the six other sets: 6-3, 6-1, 6-4, 6-0, 6-1 and 6-2.

We are hoping Radwanska can make it competitive but will be surprised if it lasts more than 70 minutes. Williams is even more the favorite given Radwanska's exertions this week.

Prediction: Williams in two