Wildcard Zheng, who upset top seed Ana Ivanovic in the third round, broke Vaidisova twice in the opening set, both times when the Czech double faulted on break point. Zheng, in turn, saved six break points in the set.

The Chinese, a former world No. 27 who came into the tournament ranked 133 after injuries, dropped a set for the first time here when Vaidisova broke her for the second time in the 11th game of the second set and went on to level the match.

However, Zheng proved the more resilient, racing away with the third set, although she needed a third match point to settle it when the Czech sent a return into the net.

The win meant that Zheng became the first Chinese player to reach the semifinals of a grand slam tournament.

The 24-year-old, who is playing at Wimbledon for only the third time, is also the first female wildcard entrant to ever reach the semifinals at the All England Club and second at any grand slam.

Elsewhere, the Williams sisters moved closer to another potential final showdown, 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 twice before been in the final, with Serena winning both in 2002 and '03.

"That would be amazing if we both were in the final," the 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.

Dementieva was a set and 5-1 up after a one-sided encounter that Petrova barely seemed to be taking part in other than to produce a steady stream of forehand errors. With the finishing post within touching distance and serving for the match, Dementieva threw the 21st seed an undeserved lifeline by double-faulting to hand Petrova her first break.

Another break by Petrova followed and what should have been an easy win suddenly turned into a more intriguing encounter.

Dementieva, the highest remaining women's seed in the draw, held two match points in the second-set tiebreak but could not convert them, enabling Petrova to force a decider.

It was a situation all too familiar for Dementieva, who was knocked out of the French Open quarterfinals by Dinara Safina after squandering a match point in the second set.

"I was so close to finishing in two sets and I don't know what happened," she said. "I suddenly started to think about the French Open quarterfinals and it was difficult (to get that out of my head). It was so exhausting."