WIMBLEDON, England — When Serena Williams sits in the stands watching big sister Venus play, she's not just cheering for her sibling.

With both in the semifinals at Wimbledon, Serena also looks for some kind of edge in case the two meet for the third time in the final Saturday at the All England Club.

"She's an opponent," Serena said. "You scout your opponents."

Before playing in the final, Serena will have to get past Zheng Jie of China, and defending champion Venus will have to beat Elena Dementieva of Russia in today's semifinals. If both Americans win, they'll play in the Wimbledon final for the first time since 2003, when Serena beat Venus in the championship match for the second straight year.

"She won last year," eight-time Grand Slam champion Serena said of Venus. "But I'm not going to sit here and say she's the favorite when I'm still in the tournament. That's not me."

Serena is 8-7 against her sister overall, but a dominant 5-1 in Grand Slam finals.

"We're extremely motivated by each other, win or lose, on or off the court," said Venus, a four-time Wimbledon champion.

Yesterday, the sisters beat Sania Mirza and Bethanie Mattek, 6-4, 6-3, in the women's doubles quarterfinals, whispering tactics to each other on occasion but otherwise keeping calm despite trailing 3-0 in the second set.

"I think the doubles gets us really amped for the singles, and the next day we're all tuned and ready to go," said Venus, who has won six Grand Slam doubles titles with her sister.

Dementieva, Venus' next opponent, struggled for long periods before beating Nadia Petrova in three sets in the quarterfinals. But she's not worried about her next match.

"She's the defending champion. It's all pressure on her," Dementieva said of Venus, who is 6-0 in Wimbledon semifinal matches and 56-7 overall at the grass-court tournament. "I really have nothing to lose."

Zheng, ranked 133rd, is this year's surprise in the women's draw, reaching the last four with a run that includes a third-round win over top-seeded Ana Ivanovic, the French Open champion.

By beating Nicole Vaidisova, Zheng became the first Chinese player to reach the semifinals at a Grand Slam tournament, and is also the first wild-card entrant to reach the women's semis at Wimbledon and second at any major tournament.

"I never think I can (reach) the semifinal," said Zheng, who is playing at Wimbledon for only the third time and is 0-1 against Serena in her career after a first-round loss at the All England Club in 2004.

"I hope I can do better when I'm playing her," Zheng later said through a translator. "And, of course, I also wish to win. But for now I just want to simply keep my hope as simple as possible."

Serena won't be taking the 24-year-old Chinese player lightly.

"I've been watching her play," Serena said. "I think she's doing a fabulous job and I don't think it's luck."

Serena and Venus have won six of the past eight Wimbledon titles. Neither has dropped a set at Wimbledon so far, and their power games have set them apart from the rest of the women's field.

"I've been working really hard lately. I'm just waiting on the results to come," Serena said. "I deserve this, because I don't think anyone's been working harder than me, except for maybe Venus. I mean, that girl works even harder than I do."

If they both advance and set up yet another all-in-the-family final, they'll get right back to supporting each other once the final point is played.

"We leave everything on the court," Serena said. "We're sisters the moment we shake hands."