NEW YORK, SEPT. 9 -- Pete Sampras, a 19-year-old who can serve through a chain link fence, became the U.S. Open champion today, and he didn't even know it. Sampras upset Andre Agassi, 6-4, 6-3, 6-2, almost casually, but sincerely, with an expression of wonder at his newfound ability. When he delivered his 100th ace of this tournament, he raised a standing ovation inside Louis Armstrong Stadium.

Sampras is the youngest men's U.S. Open champion, and possibly the least affected winner ever, a doe-eyed, awkwardly mannered, grimacing teenager whose first word on collecting the $350,000 check and the silver urn was a paused, considered "Umm. . . . " But as a player he is mature and eloquent, routing fourth-ranked Agassi, 20, and seizing the mantle of America's greatest, freshest talent.

"This is unbelievable," he murmured.

At 19 years 28 days, Sampras replaced Oliver S. Campbell as the youngest men's victor. Campbell won the 1890 title at 19 years 6 months 9 days. But 12th-seeded Sampras's performance here was far beyond his years: He upset third-seeded, three-time champion Ivan Lendl in a five-set quarterfinal, four-time winner John McEnroe in a four-set semifinal, and today overwhelmed Agassi in 1 hour 42 minutes with a serve that registered 124 miles per hour.

Sampras aced Agassi 13 times to bring his total over the fortnight to the century mark. He completed the match with a breathtaking display of force, breaking Agassi at love for 4-2, then holding at love with a pair of aces, one down the middle and one wide. That two-game, eight-point sweep completely disheartened Agassi, who then lost his serve a fifth time for the match, with two feeble ground strokes into the net.

"It's disappointing when you see somebody hold up a trophy you wanted," Agassi said. "But it's not like I lost it. I got my butt kicked. That's about it."

The mere presence of Agassi and Sampras in the final guaranteed several things, including the first American champion since McEnroe in 1984. It also assured a first-time winner, as these ingenues were all that was left after a frenzy of upsets. Agassi had beaten second-seeded, defending champion Boris Becker in the semifinals. Top-seeded Stefan Edberg was stunned in the first round by an obscure Soviet.

Not since 1977 had the three top men's seeds failed to win, allowing No. 4 Guillermo Vilas his title. To find a winner seeded lower than No. 12, you had to go back to 1966 and unseeded Fred Stolle.

"I'm going to take a couple of days to try to realize what I did," Sampras said.

It is also the first time since 1985 that four men won the four Grand Slam title: Lendl winning the Australian Open, Andres Gomez (defeating Agassi) the French Open, and Edberg Wimbledon. Combined with Gabriela Sabatini's win over Steffi Graf in Saturday's women's final, it represents the first time since 1966 that eight players divided the men's and women's majors.

But there are more irrestistible features of this final than just trivia. It was a TV match, a scenario played out between immigrant children, in the first all-American affair since 1979 when McEnroe beat Vitas Gerulaitis. Sampras and Agassi were different yet similar, a contrast in styles and strategies. Agassi was more experienced, appearing in his second Grand Slam final, but also was more anxious and pressured, while Sampras showed an almost unconscious calm in his major final debut.

"I had absolutely no nerves," he said. "It was great. I had a great time. He looked real tentative. I thought he would go for more. He was letting me dictate play and that was the difference."

There were considerable expectations heaped on Agassi, largely by himself. The son of a transplanted Iranian who's now a Las Vegas showmaster, he has been a self-proclaimed prodigy since infancy. He played the role of American hope to the hilt, calling the U.S. Open the tournament his career revolves around.

"When somebody is beating you like he was, there's not a lot you can get frustrated about," Agassi said. "It's easy to get discouraged, thinking I had a chance to win two Grand Slams this season. But I'll look at it as an indicator of things to come."

Sampras couldn't be more different from Agassi in comportment, a quiet upstart. He is a neat, well-clipped young man of Greek origin, born in Potomac, Md., and raised in Rancho Palos Verdes, Calif. His parents at one time did not even know how to keep score in a tennis match, and cannot bear to watch him play. Too nervous, they were not here.

On the court, it was a matter of Agassi's power baseline strokes, the passive aggression of his vicious returns and passing shots, against Sampras's deceptively velvet serve-and-volley conventionalism.

Sampras's numbing serve was the difference. Agassi could barely pick up -- much less return -- the thudding deliveries, with just three break point opportunities, none until the third set. It was strength against strength, and Agassi wilted. Sampras won 92 percent of the points on his first serve, and had 12 service winners in addition to his aces.

Demoralized by his inability to make a dent, Agassi also began losing the baseline rallies. For Sampras not only displayed a killing efficiency with his serve and volley, he also showed a withering force on his ground strokes, with 27 winners. Agassi hit just 10, to 28 unforced errors. And Sampras doesn't even use a supercharged, wide-body racket like Agassi does.

"When I'm serving that well, I think it puts a seed in the opponent's mind, that if he plays one bad game the set could be over," Sampras said.

That is exactly what happened. "I was completely on the defensive," Agassi agreed. Sampras lost only three points in five service games in the first set. Three times he held at love. Agassi meanwhile struggled constantly against Sampras's encroaching on the net, and the set was decided by a single break.

It came in the third game, a lengthy slugging duel. Sampras got break point with a low forehand cross, following it to the net and forcing Agassi to rush a backhand into the net. Agassi then overhit a forehand deep to give up the game, Sampras taking a 2-1 lead.

When Sampras held serve at love -- and did so with three bludgeoned aces in a row -- the tone of the match was set.

"When the guy is serving 120 miles an hour and hitting the lines, there isn't much you can do," Agassi said.

The second set followed the pattern. Sampras lost just five points in four service games, and broke Agassi in the fifth and ninth games. He killed a high, ball-deadening forehand down the line and then made a volley combination, a twisting backhand and a lunging, picked-off forehand winner, to take a 3-2 lead. He broke Agassi again for the set, with a driving backhand return winner down the line that sounded like an ax sinking into a tree.

Agassi had chances in the third set, when Sampras's serve temporarily lost some of its hurt, with two break points in the opening game and another in the third. But Sampras held both times and went on to break Agassi at love in that sixth game for a 4-2 lead, with a smooth, measured backhand winner down the line. The match was effectively over.

"That was a good, old-fashioned street mugging out there," Agassi said. "That's what it was. I gave it my best shot. That's it."