ISTANBUL, Turkey - World No.1 Victoria Azarenka fights for survival against Li Na in one of two winner-take-all encounters on the final day of Round Robin action at the TEB BNP Paribas WTA Championships - Istanbul 2012.
Friday, Round Robin
 Maria Sharapova (RUS #2) vs.  Samantha Stosur (AUS #9)
Head-to-head: Sharapova leads 10-2
Maria Sharapova may have secured her passage into the semifinals already, but there is still plenty to play for in her final Round Robin match with Samantha Stosur.
First thing on the agenda is the not-so-small matter of group standings; a win over Stosur will see her clinch first spot and avoid a semifinal meeting with the red hot Serena Williams. Secondly, after Victoria Azarenka's defeat on Thursday, Sharapova knows if she keeps winning there is still a chance of securing the much-coveted year-end No.1 ranking.
Stosur, meanwhile, is playing for pride. But that is something the Australian has plenty of and after an encouraging end to the season, she will be keen not to go out with a whimper. The players have met 12 times before, and although Sharapova has had by far the better of these, Stosur can take comfort from a recent victory in Tokyo.
 Agnieszka Radwanska (POL #4) vs.  Sara Errani (ITA #7)
Head-to-head: Radwanska leads 5-1
Both Agnieszka Radwanska and Sara Errani know that victory in their meeting on Friday will see them through to the semifinals at the WTA Championships for the first time.
Despite losing, Radwanska will be buoyed by her performance level in her last match, when she pushed Maria Sharapova all the way in a three-set epic. Offering further encouragement is a head-to-head record that stands at 5-1 in her favor and includes a one-sided win this year in Madrid.
Errani, though, sent out a message of her own with a belligerent three-set win over Samantha Stosur in her second match. Like, Radwanska, the Italian often relies on court craft over brute force and Friday's encounter is sure to feature plenty of easy-on-the-eye tennis as well as a nervy moment or two.
 Victoria Azarenka (BLR #1) vs.  Li Na (CHN #8)
Head-to-head: series tied 4-4
Victoria Azarenka's defeat to Serena Williams means that her future in the competition - and as World No.1 - is still very much in the balance. The good news is that both are still in her hands. The bad news is that next she takes on an opponent with just as much to play for, and one who has a track record of rising for the big occasion.
Li Na's impressive victory over Angelique Kerber breathed life into her campaign and, after an untidy first outing against Williams, saw her back to her destructive best. More of the same will be required not just to keep Azarenka on the back foot, but also to penetrate her watertight defenses.
With four victories in their eight encounters - including twice at majors - Li knows exactly what it takes to defeat Azarenka. However, this confidence will be tempered slightly by defeats recently in Sydney, Madrid and in Istanbul 12 months ago.