DOHA (AFP) Danish teenager Caroline Wozniacki wept and hobbled into WTA Championships folklore on Thursday, relegating Serena and Venus Williams to supporting roles. World number four Wozniacki burst into tears after overcoming Russias Vera Zvonareva and crippling cramp, while collapsing to the court in searing pain to edge closer to the semi-finals of the 4.55 million dollar season-ender. Newly-crowned world number one Serena Williams, the 2001 champion, made sure of her place in the last four with a third win in three group matches, seeing off Russias Elena Dementieva 6-2, 6-4. Defending champion Venus Williams, needing a win to stay alive after two losses, saw off French Open champion Svetlana Kuznetsova 6-2, 6-7 (3/7), 6-4 and can still sneak into the semi-finals. Her fate, however, depends on the outcome of Fridays encounter between Kuznetsova and Dementieva. But the day belonged to 19-year-old Wozniacki, the US Open runner-up who defeated Zvonareva 6-0, 6-7 (3/7), 6-4 to record her second win in two matches. Wozniacki, the youngest player in the draw, ended her 2hr 48min match sobbing after collapsing in agony, her body wracked by cramp, when she was just two points from victory. I wasnt scared. I was just thinking,How can I get up from here? It didnt matter which muscle I was moving, I was cramping, said the blonde teenager who picked herself up to clinch a famous win. I wanted to reach the hands of someone. But I wasnt allowed to find my way up. I have absolutely no idea how I won. World number nine Zvonareva, the runner-up in 2008 and who was summoned to replace the injured Dinara Safina on Wednesday, failed to take advantage of her opponents plight. Wozniacki had led by a set and 5-2 before wasting two match points in the 12th game of the second set. The 25-year-old Zvonareva battled impressively, even overcoming a nasty courtside nosebleed in the second set. Wozniacki led 3-1 in the decider, but was again pinned back to 3-3 before needing treatment on her bandaged thigh. With the tie seemingly slipping away, she bravely broke again to lead 5-4 before claiming victory when Zvonareva netted a forehand. Serena Williams, who made sure of claiming the year-end world number one spot when Safina withdrew through injury, cruised past Dementieva in the pairs sixth meeting of 2009. I havent done really well traditionally at this tournament, especially the last couple of years, said Serena, who wad beaten in the round-robin stage in 2008. I was kind of used to the elimination thing. But I was just trying to do better than I did last year really and compete in all my matches, which sometimes is very hard. It was certainly tough for Zvonareva who later followed Safina out of the tournament complaining of an ankle injury. She will be replaced by second alternate Agnieszka Radwanska of Poland. Friday sees the completion of the group stage. Wozniacki leads the White Group with two wins in two and faces Jelena Jankovic who has one win and one defeat. Radwanska, with no chance of qualifying, tackles Victoria Azarenka, who also has a 1-1 win-loss record. In the Maroon Group, Serena is already guaranteed a last-four place leaving Kuznetsova and Dementieva to meet in a match which will also decide Venuss fate.