NEW YORK - Kim Clijsters already has defeated one Williams sister. Now she must play the other.

Clijsters, who beat Venus Williams in the fourth round, will take on defending champion Serena Williams, who improved to 23-1 in Grand Slam singles matches this season by beating No. 10 Flavia Pennetta of Italy 6-4, 6-3 Tuesday night.

Looking ahead to facing Clijsters, the younger Williams said: “She’s such a great person and I, like, only wish the best for her. But not in the next match.”

The 27-year-old American and 26-year-old Belgian both have 12-match winning streaks at the U.S. Open. Clijsters won the first seven en route to the 2005 title, the last time she played in New York, and now has five this year, the last a 6-2, 6-4 win against 18th-seeded Li Na of China.

“I’m glad I got through it again, stayed focused on my game,” Clijsters said. “I wanted to be aggressive and I think that’s what helped winning those important points today.”

The few important points there were in this one came midway through the second set, after Clijsters had lost a break to turn a 3-1 lead into a 4-4 tie. Li responded with four unforced errors to give away the ninth game and the match was over a few minutes later.

Clijsters became the first unseeded player to make the U.S. Open semifinals since Elena Dementieva in 2000. Clijsters was unranked because she hadn’t played enough tournaments in her comeback to get on the board, but she’ll be in the low-50s or better when the next rankings come out.

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As efficient as she has been — moving better now than she did when she was constantly battling injuries toward the end of her last stint — her run through this tournament might also be seen as a statement about the state of women’s tennis.

Serena Williams is the only top-five seed left. Three of the players on the opposite side of the draw — the “Melanie Oudin side” — are ranked 50 or higher, joined by No. 9 Caroline Wozniacki. All are playing in their first major quarterfinals.

“I saw her when she came back in her first tournament,” Li said, referring to Clijsters. “I knew she was at a high level. She’s much stronger than other girls, so I knew, if she was going to come back, it must be a strong comeback.”

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