NEW YORK(AP) Serena Williams finally acknowledged what has seemed rather obvious for quite some time: There are four tennis tournaments that truly matter to her.

``Absolutely. I really play for the Grand Slams,'' Williams said Thursday, a day before facing Kim Clijsters in the U.S. Open semifinals.

Whenever I'm playing elsewhere, I really try. But for me, the Grand Slams - I give it that extra oomph,'' Williams added.It's where you can leave history and leave a mark, and that's kind of how I look at it.''

She's certainly left her mark lately at major championships, winning three of the past four to raise her career haul to 11 Grand Slam singles titles. The other three women left in the field at Flushing Meadows own a combined total of one: Clijsters won the 2005 U.S. Open.

The Belgian hadn't played in the event since - she was injured in 2006, then walked away from the game in early 2007. During her 2 1/2-season sabbatical, Clijsters found it tough to follow her sport closely, particularly once her daughter was born.

I only just started watching tennis at the start of this year, to be honest,'' said Clijsters, trying to become the first mother since 1980 to reach a major final.I didn't really have that much time with a baby running around, and I was happy just to sleep when she was sleeping.''

Still, Clijsters does not need a TV or a DVD to know how it will feel to have Williams on the other side of the net Friday. They last played each other in 2003, yet Williams leaves quite an impression.

She's always been able to step it up at the Grand Slams,'' Clijsters said.I've seen her play here, and she has that face where she's, like, 'OK, I'm here to do business.'''

Dating to the start of last year's U.S. Open, Williams' win-loss ledgers at major tournaments vs. lesser tournaments are striking:

-At Grand Slam events over that span, Williams is 30-1, for a .968 winning percentage, including a 10-1 record against top-10 players.

-At all other events, Williams is 21-11 (plus one walkover), for a .656 winning percentage, including a 3-5 record against top-10 players.

She has three titles from the Slams in that time - and zero titles from the non-Slams.

The No. 2-seeded Williams has been dominant through five matches in this U.S. Open, the only woman who hasn't lost a set. Plus, she and her older sister Venus will play for the women's doubles title Sunday after winning their semifinal Thursday.

Don't think for a moment that Williams is worried about not having much of a scouting report to rely on when it comes to Clijsters, who is playing in only the third tournament of her comeback and arrived at the U.S. Open without a ranking.

Williams figures she can glean all she needs to know when they step out on court in Arthur Ashe Stadium.

That's how I play my whole career,'' Williams said.I try to see in warmup what they do and go from there.''

While Williams-Clijsters features two women in their mid-20s who have played in multiple Grand Slam finals and been ranked No. 1, the other semifinal has quite a different cast: No. 9-seeded Caroline Wozniacki of Denmark and 50th-ranked Yanina Wickmayer of Belgium are both 19, and neither had played in so much as a major quarterfinal until Wednesday.

Wozniacki's 6-2, 6-2 victory over 17-year-old American Melanie Oudin made her the first Danish woman or man to reach a Grand Slam semifinal. Wozniacki's progression is not entirely out of the blue, though: She leads the tour in match victories this season with 61, and she won a tournament on hard courts in New Haven, Conn., the week before the U.S. Open.

Wickmayer, meanwhile, owned a 1-6 career record at Grand Slam tournaments before last week.

When you get to the third, fourth round, you start surprising yourself,'' she said.But actually, I've been staying pretty calm.''