NEW YORK, SEPT. 8 -- Gabriela Sabatini of Argentina, who was said to be incapable of a major victory, won the U.S. Open today. The 20-year-old, known more for her physical beauty and looping topspin than her ability, defeated the No. 1 player in the world, Steffi Graf of West Germany, with an animation she never had shown before, 6-2, 7-6 (7-4), in their final at the National Tennis Center.

The once-unassailable Graf was not only vulnerable today, she was an open wound and a weary, pursued champion. Sabatini created match point in the tiebreaker with a twirling backhand drop volley that Graf could not retrieve. Graf then punched a forehand that caught the net cord and became a perfect high target for Sabatini's crushed forehand that ticked just inside the line. Graf turned her back to the net and stood for a long moment, her chin on her chest and her hands on her hips, as Sabatini exulted.

"I can't believe I won this tournament," Sabatini said. "It's the biggest feeling. I have no words to explain this emotion."

So much for the theory that Graf, 21, who had claimed nine Grand Slam events in the last three years, including her 1988 sweep of the Australian and French opens, Wimbledon and the U.S. Open -- is invincible. This season she won just the Australian Open, victimized by upset-minded rivals who have caught on to her game. She has been wracked by poor health and personal difficulties, and subject to the mysterious, plaguing inconsistency she suffered this afternoon.

"Today I had nothing," she said.

Graf lost the French Open to third-ranked Monica Seles, 16, and was upset in the Wimbledon semifinals by Zina Garrison while Martina Navratilova went on to her record ninth title at the All England Club. But Graf had seemed poised to win her third straight U.S. title after Seles and Navratilova were upset in early rounds, allowing fifth-seeded Sabatini nearly free entrance to the final.

Graf had beaten Sabatini 18 times in 21 meetings. The Argentine represented the lowest seed to win the Open since Virginia Wade defeated Billie Jean King in 1968. This is the first time since 1981 that four different players won the four legs of the Grand Slam. And Sabatini's victory perhaps signified the beginning of what observers had predicted for years, that she and Graf could be the rivals of the 1990s, inheritors of the two-decade battle Navratilova had with Chris Evert. And that women's tennis would someday get deeper and more interesting.

Sabatini played a diverse, strategic match, nullifying Graf's notorious pace with massive topspin that pushed Graf well beyond the baseline. She alternated those strokes with low slices, and attacked the net discerningly. She made 26 winners to Graf's 18. In the meantime Graf sprayed 40 unforced errors.

That and Sabatini's pressing were what prevented Graf from extending the match. It has been typical in their rivalry for Sabatini to take a set, only to fold in the third, with either a mental or a physical breakdown. So when she broke Graf's serve twice consecutively to begin the match with a 4-0 lead, and then failed to close out the second set despite serving for the match at 5-4, she suspected that if she didn't end it soon the momentum would turn. Sabatini had played two matches of over two hours to reach the final; Graf was the fresher player with a 54-minute, straight-set defeat of Arantxa Sanchez Vicario in the semifinals.

"I thought about it," Sabatini said. "That's why I say {in the second set}, I have to win here. I have to win in two sets because it's like it always happens, the same thing when we go to a third set sometimes I get tired."

Sabatini broke Graf's serve in the ninth game when the West German made a desperate backhand volley but floated it wide at an angle, and then cut a backhand slice to the corner awry. But Graf, struggling to remain in the match, lifted her game temporarily to break back.

A big forehand return at Sabatini's feet forced a jerked backhand into the net to put her down, 0-30, in the next game. Another driven forehand forced her lunging backhand volley deep for double break point. Graf pounded an overzealous forehand wide on the next point, but battered Sabatini in a baseline rally that ended with a windmilling backhand deep. Sabatini was thus broken and the set was even at five games each.

"After the first set I just told myself to keep the ball in play more," Graf said. "Then when she was serving for the match at 5-4, I told myself to come on, try and hit it harder. That worked for a few points. But then at some moments, she hit better shots than I did."

Graf lost two set points as Sabatini served in the 12th game. She made yet another error, a floating backhand deep, and Sabatini saved the second with a reflexive forehand volley winner from her waist. They headed into the tiebreaker.

Graf seized a 3-1 lead, guessing right with a forehand pass, and then jabbing a forehand volley. But Sabatini swept the next four points, the most spectacular a monstrous topspin forehand winner to the corner with almost as much force as Graf's famous stroke, for a minibreak of serve to bring it to 3-2. When Graf shoveled a forehand half volley into the net to trail by 5-4, Sabatini sensed victory.

She made her spinning, acrobatic backhand volley for match point on a murderous Graf forehand. Then came that anticlimactic final point.

The victory surprised Sabatini as much as anybody in the stadium. Although she is only 20, she has been a pro since she was 15. This was just her second Open final, defeated by Graf here in 1988, losing a one-set lead.

"She deserves to win a Grand Slam, she's been around a long time," Graf said. "She's had her chances. She was always a good fighter, and she stayed in against me. This was a very good final, and she deserves to win it."