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 tournament . Third seed Agnieszka Radwanska is out after losing to France 's Angelique Martin . China 's World No . 5 depending on the result of Friday 's other White Group clash between Jelena Jankovic and Sara Errani . Li Na wins again in other group , faces Victoria Duval 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 tournament . Third seed Agnieszka Radwanska is out after losing to France 's Angelique Martin . China 's World No . 5 Li Na wins again in other group , faces Victoria Duval in decider .