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.

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

Zheng is playing at Wimbledon for only the third time. Even so, 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 on Saturday, they'll re-establish their family ties as soon as the final point is played.

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