Belarusian Victoria Azarenka advanced to the semifinals of the WTA Tour Championships in Istanbul and locked up the year-end No. 1 ranking for the first time with a 7-6 (7-4), 6-3 victory over China's Na Li.

The 2012 Australian Open champion won a tight first set before pulling away in the second set to take Friday evening's match in one hour and 39 minutes and finish second in the Red Group behind world No. 3 Serena Williams, who continued her demolition of the field on Saturday.

The Belarusian will look to return to the final of the year-end event for the second-straight year when she takes on world No. 2 Maria Sharapova of Russia in Saturday's second semifinal.

Sharapova emerged undefeated from the White Group after breezing past Australia's Samantha Stosur 6-0, 6-3 on Friday in one hour and five minutes.

The winner of the Sharapova-Azarenka match will have the daunting task of facing Williams, who brushed aside Poland's Agnieszka Radwanska in Saturday's first semifinal 6-2, 6-1.

The American has not lost a set in four matches, including a 6-4, 6-4 win over Azarenka on Thursday.

Radwanska, who also lost to Serena in this year's Wimbledon final, was battling fatigue after defeating Italy's Sara Errani 6-7 (8-10), 7-5, 6-4 on Friday in three-and-a-half hours, the longest three-set contest in the tournament's history.

The tournament's defending champion, Czech Petra Kvitova, withdrew from the tournament due to a viral infection after losing her first match to Radwanska in straight sets. EFE