Venus and Serena into Wimbledon semifinals

(CNN) -- Holder Venus Williams moved into the Wimbledon semifinals on Tuesday when she beat Tamarine Tanasugarn 6-4 6-3, firing family ambitions of a third title showdown against sister Serena.

The American was not at her sharpest on Court 1 but was still too good for the Thai, who was playing in her first Grand Slam quarterfinal.

Twice former champion Serena later overwhelmed Poland's Agnieszka Radwanska 6-4 6-0 on Centre Court.

"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."

Serena now encounters Zheng Jie who became the first Chinese player to reach the semifinals of a Grand Slam event by beating Nicole Vaidisova 6-2 5-7 6-1.

The 24-year-old Zheng is also the first female wild-card entrant to reach the semifinals at the All England Club and second at any Grand Slam.

Venus faces fifth-seeded Elena Dementieva who advanced to last four for the first time by beating Russian compatriot Nadia Petrova 6-1 6-7 (6) 6-3.

Dementieva, the highest seeded women's player left in the draw, was broken for the first time at 5-1 in the second set when serving for the match.

Petrova then saved two match points in the tiebreaker, winning it 8-6, before Dementieva took a 4-0 lead in the third set Tuesday and held on for a clash with four-time champion Williams.

Venus, who has not dropped a set all tournament, was limping slightly at the end of the match with a hamstring problem.

"It feels a little bit tight," she said. "I'll be fine. I'm a big girl. I can deal with it."

The top four in the women's singles were knocked out before the quarterfinals for the first time in Grand Slam history this year, but Williams never looked like being upset by Tanasugarn.

She is now just one victory away from making it seven Wimbledon finals in the last nine years.

Tamarine, ranked 60th, was the first Thai to reach a Grand Slam quarterfinal and the 31-year-old gave a gutsy display against an opponent she had not beaten in six meetings.

The Thai, the oldest player left in the women's singles, started nervously, allowing Venus to break in the third game.

Two unforced errors gifted her an immediate break back but the former world number one broke again and, when her opponent wasted six break points in the next game, the match turned in Williams's favor for good.

She served out the set and broke in the first game of the second set to take control with little trouble.

Another break in the ninth game was enough to seal the match and keep Williams on course for the final.

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 against Petrova, but 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."