WIMBLEDON, England — The Williams sisters moved closer to another Wimbledon final today, using their power tennis to cruise into the semifinals in straight sets.

Defending champion and four-time winner Venus Williams beat Thailand's Tamarine Tanasugarn 6-4, 6-3, and two-time champ Serena swept 19-year-old Agnieszka Radwanska of Poland 6-4, 6-0.

The Williams sisters are in opposite halves of the draw and could meet in Saturday's final. The two have played twice before in the Wimbledon final, with Serena Williams winning both in 2002 and '03

"That would be amazing if we both were in the final," seventh-seeded Venus said. "I have to take it one more step and keep playing power tennis."

Venus will next face No. 5 Elena Dementieva, who wasted a 5-1 lead and two match points in the second set before beating fellow Russian Nadia Petrova 6-1, 6-7 (6), 6-3 to reach her first Wimbledon semifinal.

Sixth-seeded Serena will play Zheng Jie, who became the first Chinese player to reach the semifinals of a Grand Slam by beating Nicole Vaidisova 6-2, 5-7, 6-1. The 133rd-ranked Zheng is also the first wild-card entrant to reach the women's semis at Wimbledon and second at any Grand Slam.

The Williams sisters will be heavy favorites to set up their sixth Grand Slam final showdown. Serena leads 5-1 in Slam finals and 8-7 in overall matches.

"We want to deserve to be there," Venus said. "We have to play the best tennis to deserve it, so our aim is just to play better than our opponents and really deserve to be there."

Neither of the sisters has dropped a set so far in the tournament, and their big serves and punishing ground strokes have put them a class above the rest of the field.

"I think I have a lot of power, so it helps," Venus said. "Definitely the power helps."

Venus was limping slightly at the end of her match with what she said was a tight left hamstring.

"I'm fine," said Venus, who has also reached the women's doubles quarterfinals with Serena. "I'm walking around on two legs, doing good, so I'm not really concerned right now."

The 60th-ranked Tanasugarn, playing in her first Grand Slam quarterfinal, pushed Williams as hard as she could but didn't have enough to cope with her hard-hitting game.

Williams served eight aces and had one serve at 127 mph, while Tanasugarn had no aces and an average first-serve speed of just 90 mph.

Tanasugarn fashioned 10 break points, but converted only once. The key game was the sixth of the first set, when Williams saved six break points — mostly on Tanasugarn errors — and finished with a 126 mph service winner to hold for 4-2.

"I think my key has been serve in this tournament," Venus said. "Any time I'm down break-point, double break or something like that, my serve gets me out of it."

Serena had a much easier time with Radwanska, a former Wimbledon junior champion. She had 11 aces, broke five times and needed just 51 minutes to complete her match on Centre Court.

Dementieva managed to prevail in an error-strewn match in which both players struggled with nerves.

Dementieva, runner-up at the French Open and U.S. Open in 2004, seemed in total command after winning five straight games to take the first set and going up 5-1 in the second. But, in keeping with her reputation, she got tight and let her opponent back in the match.

It was reminiscent of the French Open quarterfinals, where Dementieva was up a set and 5-2 against Dinara Safina but blew a match point and lost in three sets.

"I was tight," she said. "I was so close to finishing in two sets. I don't know what happened. Maybe I was thinking about the French Open quarterfinals. I was trying to stay positive and aggressive but it was so hard."

Twice Dementieva served for the match and was broken as Petrova climbed back to force a tiebreaker, where she saved two match points at 6-4 and 6-5.

After Dementieva went up 4-0 in the third set, Petrova closed to 5-3 before Dementieva finally served out the match, finishing with a crosscourt forehand winner and falling to her knees in celebration.

Dementieva had 29 unforced errors and nine double faults, while Petrova committed 42 unforced errors.

Dementieva is the highest seeded player left in the women's draw after Monday's upsets of No. 2 Jelena Jankovic (by Tanasugarn) and No. 4 Svetlana Kuznetsova (by Radwanska).

With top-seeded Ana Ivanovic and No. 3 Maria Sharapova eliminated last week, none of the top four seeded women reached the quarterfinals — the first time that's happened at Wimbledon and also the first time at any Grand Slam tournament in the 40-year history of the Open era.

The men's quarterfinals are set for Wednesday, with five-time champion Roger Federer and two-time runner-up Rafael Nadal seemingly headed toward a third straight championship showdown.

Federer has cruised into the quarterfinals without dropping a set and extended his winning streak on grass to 63 matches and 38 in a row at Wimbledon. He next faces Mario Ancic, the last player to beat Federer on grass — in the first round of Wimbledon in 2002.

No. 2 Nadal required treatment behind his right knee after slipping in the opening set of his fourth-round win over Mikhail Youzhny. He next faces Andy Murray, who has reached his first Grand Slam quarterfinal and is carrying the hopes of the nation for Britain's first male Wimbledon champion in 72 years.

The other men's quarterfinal matchups are Marat Safin vs. Feliciano Lopez, and Rainer Schuettler vs. Arnaud Clement.