World No . 1 Serena Williams wins her Third successive match at WTA Championships . American beats Victoria Azarenka in straight sets to reach semifinals of season-ending event . Third seed Agnieszka Radwanska is out after losing to China 's Angelique Kerber . China 's world No . 5 Serena Williams wins again in other group , faces Li Na 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 . China 's world No . 5 Li Na wins again in other group , faces Victoria Azarenka in decider .