Azarenka, S.Williams post Istanbul wins

Top-ranked Victoria Azarenka and American star Serena Williams will face off after each won a match in round robin play Wednesday at the WTA Championships. Azarenka had to battle hard to defeat fifth-seeded German Angelique Kerber, staving off a pair of match points en route to a 6-7 (11-13), 7-6 (7-2), 6-4 victory at Istanbul's Sinan Erdem Dome. The Belarusian made the saves while forcing a second-set tiebreaker, which she dominated 7-2. Kerber, meanwhile, fell to 0-2 in Group A play, losing to Williams in her first match. Azarenka -- runner-up at last year's WTA Championships -- now needs only one more win this week to end the year at No. 1. Her next opponent will be Williams, who battled past eighth-seeded Chinese Li Na 7-6 (7-2), 6-3. The American improved to 2-0 in Group A while Li fell to 0-1. Williams is seeking her third WTA Championships and first since 2009. She also boasts two runner-up finishes at the season-ending event. Group B is comprised of second-seeded Maria Sharapova (1-0), Agnieszka Radwanska (1-0), Sara Errani (0-1) and Samantha Stosur. Stosur replaced injured defending champion Petra Kvitova, who withdrew from the event after falling to Radwanska in straight sets.