Low-flying stunt planes and booming cannon fire had Kim Clijsters and Vera Zvonareva a little rattled during their quarterfinals. Come the Australian Open semifinals, they'll have other distractions to contend with.

Clijsters and Zvonareva meet in a rematch of the last U.S. Open final on Thursday, after No. 1-ranked Caroline Wozniacki and Li Na have played in their semifinal.

The 20-year-old Wozniacki has already guaranteed she will retain the top spot in the rankings by advancing this far, but is still out to prove she can win a Grand Slam.

Zvonareva shares Wozniacki's problem. She has reached No. 2 without the crowning achievement of a major trophy, and is one step away from her third straight Grand Slam final.

Third-seeded Clijsters has made it at least this far in all four majors but gone on to complete the job in just one, the U.S. Open, albeit three times. If she beats her Russian rival, she will also take her spot as the No. 2-ranked player.

Li, the outsider at No. 9, has equaled her career best by returning the semifinals here for the second straight year.

All she has to deal with is the small matter of the hopes of 1 billion Chinese seeking their first Grand Slam singles hero.

On their paths to the semifinals, each player has tried to shrug off the prospect that their potential will go unfulfilled. Their focus each time they walk out on court, they say, is only the match at hand.

As the trophy comes within sight and the competition gets tougher, the pressure builds.

Clijsters, 27, has perhaps the least to burden her run. She has her achievements: three U.S. Open titles, the No. 1 ranking, and the admiration of everyone for swapping the tennis tour for the real world then returning to her top form after getting married and having a baby.

"I remember when I first became No. 1 it was like, 'Wow! to be No. 1 in the world!'" Clijsters said Wednesday. "When you actually get to it, It's like, 'Oh, that's it?'"

If that was a dig at Wozniacki and Zvonareva, it was a gentle one.

"We have No. 1 (and) 2 still in, and Li Na has been playing really well," the Belgian said after beating Poland's Agnieszka Radwanska 6-3, 7-6 (4) in a quarterfinal Wednesday that had military planes flying overhead for the Australia Day holiday. "It's a great two semifinals and it will be tough."

Clijsters hit the No. 1 ranking in 2003, the year she reached the finals of the French and U.S. Opens and the semifinals at Melbourne Park and Wimbledon. She won the U.S. Open in 2005, then back-to-back titles in 2009 and 2010 after her return to tennis.

Clijsters has beaten Zvonareva in six of the nine matches they have played, but only once in their past four meetings, including the U.S. Open final last year.

Zvonareva may be helped by a possible injury to Clijsters — a partial tear of a muscle in her upper left leg was bothering her more than she originally thought and that it was a concern,

"I took some painkillers and anti-inflammatories before the match, but I still felt a little pain with the muscle," Clijsters told Belgian journalists. "When I land with my serve and even move or land on my left leg, it pains my muscle."

Zvonareva, 26, reached her second major final last year at Wimbledon, where she lost to Serena Williams, who is absent from the Australian Open because of a foot injury. Sister Serena withdrew in the third round with an injury.

The Russian defeated the Czech Republic's Petra Kvitova 6-2, 6-4 despite in their quarterfinal Wednesday distractions from cannons booming as part of nearby Australia Day celebrations, and the collapse of a spectator in the stands. She says she has the experience now to take this tournament in her stride.

"I really don't think (about) what is going on around the tournament, what newspapers say, what people say," she said. "To be honest with you, I don't really care. I'm just trying to do my best here, and I know every single match is very tough."

Li is on a 10-match winning streak that has included beating Clijsters in the final of the Sydney warmup event earlier this month, and is chasing the first Grand Slam finals berth for a Chinese player.

Against Wozniacki, the hard-driving baseliner has won twice in three matches, including sending the Dane out of last year's Australian Open in the semifinals.

"She hasn't lost a match this year yet, she won in Sydney and she is on a roll," Wozniacki said. "I think Li Na has improved a lot, but I also believe I have. Just going to go in there, fight for every point, and just see what's going to happen."

Li said she'd leave her coach to study Wozniacki's game, preferring to concentrate on her own.

"I mean, I never think about how the opponent play," Li said. "So you just play your tennis on the court."