Sanchez Vicario All Business : U.S. Open women: In quarterfinal, she beats error-plagued Graf for only the third time in 18 matches.

Every once in a while in a Grand Slam event, Arantxa Sanchez Vicario seems to pop up from nowhere and tweak somebody’s nose. She smiles, raps a few two-fisted backhands into the corner and knocks women’s tennis on its ear.

It’s hard not to like Sanchez Vicario, an impish, round-faced 20-year-old from Barcelona, but looks can be deceiving.

Don’t be fooled by her baby-faced innocence--she is capable of dispensing both mirth and mayhem as she travels on her merry way through tennis’ world of the unexpected.

On a sunny Wednesday afternoon at the U.S. Open, Sanchez Vicario struck again, this time plugging one of her favorite victims in a straight-set upset of second-seeded Steffi Graf, 7-6 (7-5), 6-3, in the quarterfinals.

Sanchez Vicario, seeded fifth, defeated Graf for only the third time in 18 matches, but those three have been doozies: the 1989 French Open final, the 1991 French Open semifinals, and now this.

And how did “this” happen, Arantxa?

“I play great tennis,” she said. “I deserve to win and I won.”

There you have it. Of course, it didn’t hurt that Graf, a two-time winner of this event, played loosely and erratically, unable to change either her game plan or the number of times she knocked balls outside the lines.

She had five double faults, twice lost leads in the tiebreaker, lost her serve four times and had more than twice as many unforced errors as winners. Otherwise, it was vintage Graf, who was 5-0 against Sanchez Vicario this year.

Each point Sanchez Vicario won in the tiebreaker was the result of an unforced error by Graf.

“I had my chances, I just, I don’t know, I just didn’t take them,” Graf said. “It was difficult to do anything because I wasn’t playing good enough to really hurt her.

“I am just disappointed the way I played this tournament. I think I played not up to the standard that I wanted. I just didn’t feel comfortable about what I was doing.”

There was little comfort to be found in the other women’s quarterfinal, a sister act in which 25-year-old Manuela Maleeva-Fragniere defeated her 17-year-old sister, Magdalena, 6-2, 5-3, when the younger Maleeva retired with a thigh injury.

The elder Maleeva will face Sanchez Vicario in a semifinal Friday--top-seeded Monica Seles meets seventh-seeded Mary Joe Fernandez in the other--but she wasn’t too happy about having to beat someone from her family to get there.

“Maggie is my baby sister and it wasn’t very much fun,” Maleeva- Fragniere said. “I am happy that I am in the semifinal and I am sad that my sister lost.”

Sanchez Vicario’s victory over Graf required only 1 hour 40 minutes of work, but hours in the planning. Chief among the tactics was to hit as often as possible to Graf’s backhand. This is the not-so-new textbook on how to beat Graf because she almost always hits a defensive slice from her backhand side.

Sanchez Vicario exploited Graf’s backhand to perfection, then watched her splatter the court with 28 unforced errors. In the second set, Graf had 21 to four for Sanchez Vicario.

“I was really concentrate very well,” Sanchez Vicario said. “I am just concentrate.”

Tennis understands.