( CNN ) -- Serena Williams became the first woman through to the semifinals of the season-ending WTA Championships on Thursday , winning her third successive match in Istanbul . The world No . 1 trounced former Wimbledon champion Petra Kvitova 6-2 6-3 in just 72 minutes to secure top spot in the Red Group . The American has now won her past 13 matches at the event , passing Chris Evert to have the second-best unbeaten streak behind Martina Navratilova 's 21 in a row from 1983 to 1987 . `` I feel pretty good -- I was really aggressive tonight , '' Williams said after extending her career record to 5-0 against the fourth-seeded Czech , firing 27 winners -- with 11 aces -- and making just 13 unforced errors . `` Petra was aggressive too , but I think my serve did really well . I 'm just glad to still be alive in the tournament -- I did n't have any expectations coming here , I just wanted to come here and enjoy myself , and that 's what I 've done . `` It just feels good to not be out of the tournament . '' However , Polish third seed Agnieszka Radwanska is out after losing 6-2 6-2 to Angelique Kerber . The German now has a 1-1 record , as does Kvitova , ahead of their winner-take-all clash on Friday . In the White Group , Li Na is `` one foot '' away from making another piece of tennis history . Already the first Chinese player to win a grand slam singles title , and the first to be ranked in the top five , she will now seek to reach the semifinals of the WTA Championships for the first time . Two previous attempts have ended in disappointment , going home with a 1-2 record on each occasion , but the 31-year-old is in touching distance this year after winning her second successive match in Istanbul . `` It was tight , but at least it 's better than the last two years , because now I 've got one foot in the semis -- but another foot still out of the door , so I have to see how it goes tomorrow , '' she said . Li faces world No . 2 Victoria Azarenka in her final round-robin match , and may still go through even if she loses -- depending on the result of Friday 's other White Group clash between Jelena Jankovic and Sara Errani . Li beat Jankovic 6-3 2-6 6-3 on Thursday in the first match this week to go to three sets , leaving Serbia 's former world No . 1 with a 1-1 record ahead of her final match against Errani -- who is out of contention following two defeats . Azarenka , who beat Li in January 's Australian Open final when she was the top-ranked woman , also has a win and a loss . The Belorussian has won their past five encounters , including at the WTA Championships in 2011 and 2012 , but has been in poor form recently with just one win in four matches since losing the U.S. Open final to Williams . Meanwhile , Grigor Dimitrov -- known earlier in his career as `` Baby Federer '' -- will face the real thing in Friday 's quarterfinals at the Swiss Indoors event . The Bulgarian , who claimed his first ATP Tour title in Stockholm last weekend , earned a clash with 17-time grand slam winner Roger Federer by beating Alexandr Dolgopolov 6-3 6-2 . Defending champion Juan Martin del Potro set up a clash with French qualifier Paul-Henri Mathieu , while Edouard Roger-Vasselin and Daniel Brands also went through to the last eight . World No . 3 David Ferrer reached the quarterfinals of his home Valencia Open , where he will play fifth seed Jerzy Janowicz , but American No . 4 John Isner was upset by Frenchman Jeremy Chardy .
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 .