World No . 1 Serena Williams wins her third successive match at WTA Championships . American beats Petra Kvitova in straight sets to reach semifinals of season-ending event . The German now has a 1-1 record , as does Kvitova , ahead of their winner-take-all clash on Friday . Third seed Agnieszka Radwanska is out after losing to Germany 's Angelique Kerber . TL;DR Li Na wins again in other group , faces Victoria Azarenka in decider .
World No . 1 Serena Williams wins her third successive match at WTA Championships . American beats Petra Kvitova in straight sets to reach semifinals of season-ending event . Third seed Agnieszka Radwanska is out after losing to Germany 's Angelique Kerber . TL;DR Li Na wins again in other group , faces Victoria Azarenka in decider .