BY LIZ CLARKE

Washington Post Staff Writer

WIMBLEDON, England — Their challengers have come in all forms, from an obscure British wild card to a former world No. 1, and from all around the globe — old Europe and new, Asia and the United States, too.

But no player has managed to win a single set against Venus or Serena Williams through five rounds of play at Wimbledon this year. And it looks increasingly as if the sisters, who have won six Wimbledon championships between them, may be each other’s only worthy foil.

The Williams sisters moved one step closer to meeting in Saturday’s championship by breezing past a pair of overwhelmed quarterfinalists today.

Venus, the four-time and defending champion, was first to book her place in Thursday’s semifinals, subduing 31-year-old Tamarine Tanasugarn of Thailand, 6-4, 6-3.

Serena was even more impressive in blasting 19-year-old Agnieszka Radwanska off Centre Court, firing 11 aces — including four in one game — en route to a 6-4, 6-0 victory. The match lasted just 51 minutes. The second set, in which Radwanska scored only eight points, raced by in 19 minutes.

Up next for Venus is fifth-seeded Elena Dementieva, a hard-working, hard-hitting Russian who is notorious for seizing up with nerves at critical moments. Dementieva’s demons reared up again today as she squandered two match points in the second set after building a 5-1 lead. She lost the tiebreak that followed but regrouped, after changing into a fresh dress, to oust fellow Russian Nadia Petrova, 6-1, 6-7 (8-6), 6-3.

But the day’s most compelling story was written by China’s Zheng Jie, who continued her improbable march to Wimbledon’s final four by toppling 18th-seeded Nicole Vaidisova of the Czech Republic, 6-2, 5-7, 6-1.

Zheng, 24, needed a wild card to enter Wimbledon after suffering an ankle injury that idled her for most of 2007 and sent her world ranking tumbling to 163rd. As half of Wimbledon’s victorious women’s doubles team in 2006, Zheng was granted the entry and has played with tremendous power and precision, knocking off three top-20 seeds in succession, including world No. 1 Ana Ivanovic.

Speaking through a translator, Zheng said she never imagined she could reach Wimbledon’s semifinals. No one in her family played tennis, she explained, and she didn’t pick up a racket until age 10. "But as soon as I start to play it, I fall in love with it," she said.

Zheng’s reward for her stirring performance, the best by an Asian singles player in Wimbledon history, is a rematch with Serena, who beat Zheng in straight sets during her run to a second consecutive Wimbledon title in 2003.

"Of course I hope I can have better achievements when I’m playing with her. And of course I also wish to win," Zheng said. "But for now I just want to simply keep my hope as simple as possible."

Zheng, who is from China’s earthquake-ravaged Sichuan province, said through the translator that she would donate her portion of her tournament winnings to relief efforts in the region. Another portion, she explained, will go to China’s national tennis association, which has funded her international travel.

Asked what tactics she would employ against Serena, Zheng said: "Serena, with no doubt, is (an) outstanding player. And so far I haven’t found any weak link about her."

While British oddsmakers have anointed Venus the favorite to win a fifth Wimbledon title, Serena has looked slightly more impressive, reducing her unforced errors and honing her attack in each round. Against Radwanska she committed just six unforced errors and 27 winners, a staggering number of winners given the match’s brevity.

And her serve was both powerful and on target. In a nearly flawless second set, Serena landed 86 percent of her first serves.

"It’s too much, you know," said a bewildered Radwanska. "She was playing too good for me today."

Serena said her serve made the critical difference. "I placed ’em well," she said in an understatement.

And while she insists that Venus is her most formidable opponent, Serena balked when asked if she didn’t consider Venus a favorite to win this year’s title.

"I’m not going to sit here and say she’s the favorite when I’m still in the tournament," Serena said. "That’s not me. I always believe I’m the favorite. I mean, I always believe I can win."

Venus shares the same belief. She limped slightly as she walked off Court 1 today but said it was hardly cause for concern — just a sore hamstring that’s to be expected after playing singles and doubles in a Grand Slam event.

It has been somewhat tricky to gauge how well Venus is playing because her opponents haven’t forced her to produce great tennis. She has yet to face a player ranked in the top 40. But Venus dismissed a suggestion that her performance, or that of Serena, had anything to do with the woman across the net.

"It doesn’t matter for us how the next player’s playing," Venus said. "We believe that we’re the best on the court at that moment. ... It doesn’t play a factor in how we feel about ourselves