DOHA, Qatar -- Serena Williams became the first player to clinch a spot in the semifinals of the WTA Sony Ericsson Championships on Thursday, defeating Elena Dementieva 6-2, 6-4 for her third straight win in the tournament's round-robin group phase.
Her sister Venus, the defending champion, stayed alive by beating Svetlana Kuznetsova 6-2, 6-7 (3), 6-4 for her first win in three matches. She needs Kuznetsova to beat Dementieva on Friday to have a chance of advancing to the semifinals.
After the match, Zvonareva, who replaced the injured No. 1 Dinara Safina on Wednesday, pulled out the tournament citing a right ankle injury. Her place will be taken by the second alternate, Agnieszka Radwanska of Poland. She will play Victoria Azarenka on Friday but has no chance of advancing.
Serena was broken to trail 2-1 in the first set but then won seven straight games to take control. She converted her third match point when Dementieva sent the ball into the net and then pumped her fist and yelled "Finally!"
Serena, who secured the year-end No. 1 ranking when Safina retired with a back injury, has failed to get out of the round-robin phase of the elite tournament for the last two years.
"I'm happy to finally do well," she said. "I'm kind of used to the elimination thing: If you lose, you're out. But I was just trying to do better than I did last year and compete in all my matches, which sometimes is very hard."
Dementieva, who rallied from a set down to beat Venus in their opening match, didn't find her stride against Serena until trailing 2-0 in the second set. The encounter failed to live up to their epic three-set Wimbledon semifinal -- billed by many as the best women's match of the year.
Dementieva won the longest game of the match after converting her fifth break point to make it 2-1, and the set went with serve until the last game.
Venus, who lost another three-setter to Serena on Thursday, broke Kuznetsova at love for a 5-4 lead in the third set and then converted her third match point when the Russian's backhand landed in the net shortly before 1 a.m. local time.
Venus dominated the first set but again struggled to put the match away, after wasting a one-set lead in both her previous matches.
"Svetlana just started to play really well in the second and third set," she said. "It's always great to win these kinds of matches."
Wozniacki needed two medical timeouts to treat her cramping left thigh in the third set and collapsed to the court sobbing and clutching both legs after netting a forehand at 30-15 in the last game. But she got back up and clinched the win when Zvonareva, who had saved two match points in the second set, netted a forehand on the third.
"I have absolutely no idea how I pulled it through, but I'm very happy about it," Wozniacki said.
It was the U.S. Open finalist's second win in the tournament, putting her on the verge of the semifinals.