DOHA (AFP) Defending champion Venus Williams reached the semi-finals of the WTA Championships without hitting a ball on Friday as organisers avoided potential embarrassment over the events scoring system. Williams reached the last four - along with Jelena Jankovic and Caroline Wozniacki - despite losing two of her three round-robin group matches. The American was assured of joining sister Serena in the semi-finals when Svetlana Kuznetsova defeated Elena Dementieva 6-3, 6-2 in the concluding Maroon Group match. On Saturday, Serena will face Wozniacki while Venus tackles Jankovic for places in the final of the 4.55 million dollar tournament. Under a complicated qualifying system, Venus pipped Dementieva for a last four place despite both players having a win apiece and with the Russian having defeated the American when they met on Tuesday. Venus went through having won more sets. In the White Group, Jankovic also made the last four, comfortably defeating US Open runner-up Wozniacki 6-2, 6-2. Wozniacki then faced the possibility of being eliminated from the tournament despite having two wins. However, the blonde Dane was saved when best friend Victoria Azarenka was forced to retire injured against Polish replacement player Agnieszka Radwanska. Azarenka had led 6-4, 5-2 before becoming severely restricted by a right thigh and back injury which needed constant attention. She was eventually forced to quit in floods of tears in an echo of Wozniackis injury-hit win over Vera Zvonareva the day before. Radwanska, who had in turn replaced an injured Zvonareva and goes home with 200,000 dollars for her days work, was leading 4-1 in the final set when the Belarussian hobbled away. I feel so sorry for Victoria, because I know how shes feeling. Its never fun to pull out with an injury, especially in such a big tournament, said Wozniacki. But Im in the semi-finals and Im happy about that. Ive also been fighting on the court. Jankovic came into her match far fresher than 19-year-old Wozniacki, the world number four. The 24-year-old Serbian had spent just 95 minutes in total on court in two matches after Dinara Safina withdrew through injury from her second tie after only 12 minutes. In a dramatic contrast, Wozniacki needed 5hr 48min to see off Azarenka and then Zvonareva, only surviving the clash with the Russian after overcoming a crippling attack of cramps. I didnt have anything left, said Wozniacki, who again played with her left thigh tightly-strapped and appeared at her news conference with her leg and stomach iced. I couldnt play like I wanted to. I had some pain in my stomach muscle, as well, so I couldnt serve, but I feel I deserve to get to the semi-finals. For Jankovic, who made the season-ending championships as the eighth and last qualifier, it was a third win in three career meetings against the Dane. I wanted to reach the semi-finals and I didnt want to get into long rallies with Caroline, said Jankovic, who admitted she had let herself down in her opening defeat to Azarenka. I only arrived on Sunday from Moscow where it was very cold and here its very hot. It takes time to get used to the conditions. It was a bad day against Victoria. But in tournaments like these its important how you finish, not how you start.