Serena Williams reaches semifinals in Doha

DOHA, Qatar (AP) - Serena Williams became the first player to clinch a spot in the semifinals of the WTA Sony Ericsson Championships after defeating Elena Dementieva 6-2, 6-4 Thursday for her third straight win in the round-robin tournament.

Williams was broken to trail 2-1 in the first set but won seven straight games to take control of the match. She moved the Russian around the court with her powerful ground strokes.

She converted her third match point when Dementieva sent the ball into the net, then pumped her fist and yelled "Finally!"

Williams secured the year-end No. 1 ranking after Dinara Safina pulled out with a back injury. Williams failed to get out of the round-robin phase of the season-ending tournament the last two years.

"I'm happy to finally do well," she said.

The win also means Venus Williams will remain in the tournament. She needs to beat Svetlana Kuznetsova in Thursday's late match to advance.

Dementieva, who rallied from a set down to beat Venus Williams in their opening match, didn't find her stride 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.

Earlier, Caroline Wozniacki overcame severe leg cramps and a tenacious Vera Zvonareva to win 6-0, 6-7 (3), 6-4.

Wozniacki needed two medical time-outs 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 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.

Wozniacki said her leg started cramping at 3-1 in the third set.

"From there, it just got worse and worse," she said. "I'm feeling better now. I got to cool down, getting some massage. ... I'm going to do everything tonight to recover to be ready for tomorrow."

Zvonareva replaced Safina as an alternate and had a dismal start, losing her first four service games. At 3-2 in the second set, she needed a five-minute medical time-out to treat a bleeding nose, and the match looked headed for a quick finish after she lost the next two games at love.

However, Wozniacki allowed her opponent back into the match as Zvonareva won the next three games and saved two match points when serving at 6-5 - the first thanks to a Hawkeye challenge after a line judge ruled that her forehand had gone wide.

The lucrative Doha tournament is the last WTA Tour event of the season for the eight top-ranked women, with the top two players from each group advancing to the semifinals. The winner earns $1.55 million depending on her results in the group phase, where each win is worth $100,000.