So observers were wondering yesterday why the good folks at Clearasil haven't signed up the ravishing Russian to peddle the popular acne cream.

Spotted around the National Tennis Center and in the players lounge, Kournikova appears to be doing battle with a bad case of acne.

Kournikova - who had red blotches on her forehead and cheek - was quickly ousted from the U.S. Open early last week.

"I couldn't believe how bad her skin was," one reporter noted, adding, "Then again, she is 20."

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Call it the U.S. Open meets rock 'n' roll.

Former Led Zeppelin frontman Robert Plant was enjoying a whole lotta tennis as the guest of John McEnroe.

At McEnroe's book signing outside Arthur Ashe stadium, the affable rock legend joined the tennis bad boy on stage.

An insider said the pair had met several years ago and were "into seeing each other again."

"Let's hear it for Robert Plant," McEnroe cajoled the huge crowd, who yelled "Zeppelin" and "sing something, Robert!"

The friendship is natural. Plant is a neophyte tennis player, and McEnroe has played guitar for his own vanity band.

"He asked me to warm up with him once in Croatia," Plant laughed.

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Heartthrob Tommy Haas has emerged as one of the premier characters of this year's U.S. Open - but yesterday, he bowed down to The Man.

The 24-year-old, third-seeded German earlier captured the spotlight with the nude photo of him and his ex-girlfriend that appeared in The Post Wednesday. Later that day, he donned a sleeveless shirt that prudish USTA officials made him change.

But in his match against Karol Kucera of Slovakia, the Deutsche dandy wore a conservative white and blue top with white shorts.