GIMELSTOB CLEANS `HOUSE' AT UCLA.

Byline: Howard Beck Daily News Staff Writer

Justin Gimelstob is young and impetuous, so it's no shocker when the 21-year-old shouts something mildly absurd like, ``This is my house!''

But when Gimelstob made that electronically amplified declaration to a packed house at the Los Angeles Tennis Center at UCLA on Friday night, no one could smirk at his exuberant boast. He had already backed it up.

Gimelstob, the former UCLA star and the world's 98th-ranked player, pulled off a stunning upset of top seed and fifth-ranked Patrick Rafter in the quarterfinals of the Mercedes-Benz Cup. And he had to boast - albeit semi-seriously - afterward.

``I was just joking around. I was just mouthing off,'' Gimelstob said an hour after defeating Rafter 6-4, 6-3. But he was serious when he said he expected the victory.

``I'm not shocked. I'm excited,'' he said. ``I've been working hard to put myself in a position to play that kind of match. It's a big win. He's a great player.''

The victory puts Gimelstob in today's semifinals, pitted against the streaking Andre Agassi, who defeated Australian Sandon Stolle earlier in the day. Agassi was the victim of Gimelstob's last L.A. upset, a year ago in the same tournament.

Which upset was the bigger thrill?

``To beat Andre tomorrow maybe,'' he said. Of Agassi, a sometime-critic of Gimelstob, the former Bruin said, ``I know he wants a piece of me.''

No one had broken service until Gimelstob broke Rafter in the 10th game to win the first set, pumping both fists toward his friends in the stands to accentuate the early lead.

He broke Rafter again in the second set en route to a 4-1 lead. Rafter eventually broke back to make it 4-3, but Gimelstob broke back immediately, taking advantage of two double-faults by Rafter. He closed out the match on his next service, though it took three attempts at match point to do it.

``He played very well,'' Rafter said. ``The conditions probably weren't as quick as I have played the last couple times. It was difficult to get much out of my serve tonight.''

He bristled at suggestions he might be shocked at the loss.

``(Gimelstob) played very well tonight. You guys are making it out like I'm gonna go home and cry about it. It's just another loss. Life goes on,'' Rafter said.

Agassi struggled early in his 7-6 (7-2), 6-2 defeat of Stolle. Why so much trouble against a clearly overmatched opponent? The shadow knows.

``When the sun was going down, there's these trees up there (casting shadows) that literally, every foot, (the ball) is going in and out of the sun. You just can't see the ball at all at that time of the day,'' Agassi said. ``When the sun went down and it became even out there, there were no shadows and I could pick up the ball and I started hitting a cleaner ball.''

In today's other singles semifinal, unseeded Guillaume Raoux of France will take on No. 2 seed Tim Henman of England. Both men advanced after grueling three-set matches Friday.

Henman needed 2 hours and 12 minutes to defeat Zimbabwe's Byron Black, a USC alumnus, 5-7, 6-1, 6-4. Raoux took 2 hours and 44 minutes to defeat Brentwood's Michael Joyce, 6-7 (7-9), 6-4, 7-5.

The soft-hitting Joyce played most of the match beyond the baseline, frustrating Raoux, who prefers a more up-tempo game.

``He's very difficult to play against here,'' Raoux said, ``because the balls are very soft and he's playing balls with no power inside. So it's very tough to hit the ball.''

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2 Photos

Photo: (1--Color) Top-seeded Patrick Rafter, upset by Justin Gimelstob, said he wasn't going to ``go home and cry about it.''

(2) Justin Gimelstob kept his mouth closed on this backhand volley, but opened it wide afterward.

Michael Owen Baker/Daily News