Russia's Maria Sharapova scored her second-straight win at the WTA Tour Championships in Istanbul, rallying for a 5-7, 7-5, 7-5 victory over Poland's Agnieszka Radwanska.

In a match between two players who won their opening White Group matches, the world No. 2 eked out the victory Wednesday in three hours and 12 minutes to clinch a berth in the semifinals.

Sharapova will square off in her final round-robin match against Australia's Samantha Stosur, who stepped in after defending champion Petra Kvitova of the Czech Republic withdrew from the event with a viral infection.

Kvitova lost to Radwanska 6-3, 6-2 on Tuesday, while Sharapova easily defeated Italy's Sara Errani by an identical score.

In Red Group action, tournament favorite Serena Williams has advanced to the semifinals after defeating Germany's Angelique Kerber 6-4, 6-1 on Tuesday and China's Na Li 7-6 (7-2), 6-3 on Wednesday, while world No. 1 Victoria Azarenka of Belarus saved two match points to edge Kerber 6-7 (11-13), 7-6 (7-2), 6-4 on Wednesday.

Williams and Azarenka will meet Thursday in a rematch of this year's U.S. Open final, which the American won.

Azarenka is one win away from clinching the year-end No. 1 ranking for the first time. EFE