NEW YORK, SEPT. 8 -- For all of Andre Agassi's postures, his artificial day-glos and the dyed blond hair, his tennis is for real. It may be the only thing about him that is, as Agassi upset defending champion Boris Becker in four sets in the U.S. Open at the National Tennis Center to reach the final today, and then knelt in prayer on Stadium Court.

In the second semifinal, 19-year-old Pete Sampras ensured the first all-American Open men's title match since 1979, ending John McEnroe's dream of winning his first Grand Slam title since 1984, 6-2, 6-4, 3-6, 6-3. For 12th-seeded Sampras, it's the first major final. McEnroe's ethereal touch game was no match for the Californian's powerful serve-and-volley effort.

Agassi, the 20-year-old from Las Vegas seeded fourth, recovered from a one-set deficit to thoroughly defeat the No. 2 player in the world, 6-7 (10-12), 6-3, 6-2, 6-3. He dealt Becker a series of lacerating strokes for 34 winners that left the West German sprawled on the court in the third set, bemoaning line calls, his own wretched 53 unforced errors, and his inability to chase down his opponent's unrelenting pace from the baseline.

"Reflecting now, I would have to say no question this match has probably done a lot for how people are going to perceive me," Agassi said. "That wasn't my concern going in, and it won't be my concern tomorrow, but I do feel maybe the world will start thinking I could really do it."

When Agassi meets Sampras Sunday it will be the first Open men's final between Americans since 1979, when McEnroe defeated Vitas Gerulaitis.

Becker departed with the knowledge that he did not win a single Grand Slam event this season, and that his play here was only fair, more an effort of willpower. His form was dull, lacking the thunderclap serves and huge strokes with which he won Wimbledon as well the Open last year.

After the first set, which represented perhaps the most engrossing and high quality tennis of the tournament, Becker fell apart. He hit just 28 winners to his 53 errors, and made just 37 percent of his first serves in the momentum-changing second set. Over the second and third sets he had his serve broken six straight times. But some of his lack of success was attributable to Agassi, who pinned him at the baseline and swung him from side to side with lashing pace. Becker could never comfortably charge the net, where he might have suffocated Agassi with his large presence.

"I haven't seen him play like that ever," Becker said. "He hit it in the corners for three hours."

The match represented an important breakthrough for Agassi. He has appeared in just one previous Grand Slam final, losing the French Open to Andres Gomez. He has long maintained that the U.S. Open takes precedence over the Australian and French Opens and Wimbledon, and he finally put his money where his mouth is. He also achieved what he has promised for so long, a move upward against the top three players.

"It is a bit surprising," Agassi said. "But I've been in the top four for a couple of years now and I'm challenging these guys every time I go out on court."

Agassi also managed to hold a lead, something he has had trouble doing before. Last season he lost the Italian Open to Alberto Mancini despite a two sets to one margin; and he dropped a famous Davis Cup match to Becker over five sets after a two-set lead. He maintains the difference is improved physical strength, bulking up considerably, and simple experience in important matches, which formerly intimidated him.

"It just felt bigger than me," he said.

But on this occasion it was Becker who failed to hold a lead, after seeming to break Agassi's spirit in the taut first set. Becker broke Agassi's serve to open the match, Agassi broke back, and they remained even until the tiebreaker. Agassi had five set points, including opportunities to break Becker in the 10th and 12th games. Becker had four set points, all in the tiebreaker, before clinching it.

"I don't think I ever played a set like that," Becker said. "Every point was good. It was unbelievable tennis."

Agassi clutched a backhand deep to lose the set point in the 10th game. Becker killed the set point in the 12th game with two massive forehands to the corner followed by a backhand volley winner.

In the tiebreak, Agassi got double set point with an inside out backhand winner down the line for a 6-4 lead. Becker killed the first with a low backhand slice crosscourt that caught Agassi halfway to the net. He smothered the second with a drilled backhand that Agassi took with a forehand volley, but pushed sloppily in the net.

Then Becker got his first set point, with a huge high forehand, but Agassi replied with a massive forehand kill of his own. A twisting serve winner gave the West German another, 8-7, but Agassi drove a forehand cross that Becker lunged at with a forehand wide down the line. When another Becker forehand went wide, Agassi had his fifth and last set point. Becker launched another twisting serve to the body that Agassi pushed into the net with a backhand for 10-10.

Another disguised Agassi backhand down the line deprived Becker of his third set point. But then Agassi made a fatal unforced error, a backhand wide to give Becker a margin of 11-10. Becker played a masterful point, rearing back and delivering a sharp flat serve to the backhand that stretched Agassi off the court and left him just one play, down the line. Becker was there to cover it with an easy, rolling forehand winner across the court.

"That was a very depressing feeling," Agassi said. "You can look at it two ways. You can let it get the best of you, or you can say, hey, that took as much out of him as it did me."

Becker's level of tennis plunged in the second and third sets. He held serve just once in the second set and not at all in the third. "That was bad," he said. He also railed against a series of line calls, screaming and jerking around the court, and finally laying down with his arms spread.

In the fifth game of the third set, Becker's high backhand down the line was called wide for 15-40. He dropped his racket and protested furiously. He worked back to deuce, but then a first-ball forehand into the net gave Agassi break point again. He double faulted, and Agassi had a 4-2 lead.

The histrionics continued in the next game. When Agassi's backhand down the line looked wide but the lineswoman corrected herself and called it good, Becker again screamed. Agasi made a smooth backhand pass for 30-0. Then Becker's backhand slice floated to the baseline and gently touched down. There was no call from the linesman, but the chair umpire ruled it deep to give Agassi a 40-0 lead. Becker fell to the ground and threw out his arms.