There came a point midway through the fourth set, a set Jimmy Arias was prepared to concede, when Yannick Noah made one of those routinely spectacular gets that he uses to defeat and demoralize his foes: a forehand block of an overhead he should never have reached. Arias retreated, whirled and hit the ball between his legs, the way Noah did two days ago.

Arias lost the point but made an important one, a lasting one.

Because Arias is precocious, because he refuses to concede there is a ball he can't reach, because he consistently does what no one expects him to, he beat Noah, the No. 4 seed, 7-6 (7-4), 4-6, 6-3, 1-6, 7-5. It was not only the best match of the day, it was the best match of the tournament.

Because Ivan Lendl is just that much older, because his game is that much more developed on the spectrum of tennis surfaces, because he has more colors to work with, he beat fifth-seeded Mats Wilander this afternoon, 6-4, 6-4, 7-6 (7-4).

They began with the same premise. Clay. They perfected their games, established their names and their turf, only to learn that the world demanded more. They are all in the process of proving and transforming themselves.

Wilander, 19, won the French Open in 1982 and learned about the expectations of being Bjorn Borg's heir. Noah, who is 23, beat Wilander in the final of the French Open this year and is learning about the expectations of being the first Frenchman to win it since 1946. Lendl had a 44-match winning streak but learned about a world that said, "Talk to us when you win a Grand Slam event."

Coming into this tournament, only Arias, the ninth seed here, the winner of the U.S. Clay Court championships and Italian Open this year at 19, faced no expectations. "I thought I had no chance in this match after the first three games," he said.

He was down, 0-3, at the time. And soon enough, he was behind, 2-5. But he battled back, forced a tie breaker, fell behind, 1-4, then won the next six points.

All night long, he made the impossible shots to prolong his impossible dream. While Noah made errors (27 forehand and 27 backhand), Arias made friends. The crowd at the National Tennis Center marveled at his forehand passing shots (22), which seem far too powerful for someone so diminutive. Noah marveled at his improved backhand passing shots (18).

Not only did he play well, he played smartly. When he fell behind in the fourth set, he conceded it. "I threw it to away to run him," said Arias, who had gone five sets only once before in his career (during the fourth round here this year, beating Joachim Nystrom, 6-0, 6-0, in the last two sets). "I knew we would be playing a fifth set. He doesn't get tired but it saved me."

Noah, who has been suffering from tendinitis in his right knee, said he moved slowly to the ball and considered not playing in the tournament. Asked how he felt about playing in such a spectacular match, he said, "I would rather play a very bad match and win."

Neither wavered in the fifth set. Noah cut down on the volley errors that betrayed him all night. He had forgotten the three set points wasted in the last game of the third set and dominated the fourth. There was no break in the fifth set (each had a 15-30 point once) until Noah served at 5-6. A searing forehand return down the line made it 0-15, a double fault 0-30. Another searing forehand return forced a volley long and made it match point.

Noah, whose first-serve percentage wavered in the first and the fifth sets, missed a first serve. The second came to Arias' forehand. He clobbered it. "I liked to see that," he said. "I thought, 'God, this is my match if I don't choke.' "

He didn't. The cross court landed fair and he hopped, skipped and jumped to the net.

Thus, he earned the right to play Lendl in one semifinal Saturday; No. 3 Jimmy Connors, the defending champion, will meet No. 16 Bill Scanlon (the upset winner over No. 1 John McEnroe) in the other. In the women's semifinals today, No. 5 Pam Shriver will play No. 1 Martina Navratilova and defending champion Chris Evert Lloyd, the second seed, will face No. 14 Jo Durie.

"I don't think I can play three great matches in a row," Arias said. "That's one. I don't think I can do it yet."

Earlier this week, Wilander said he wasn't ready to win the U.S. Open. He was being honest, not modest. Someday he will the Open, which Borg never did, and differentiate himself forever. But he must strengthen his serve, his volley and his resolve to come to the net whenever the opportunity is presented to him.

At 23, Lendl is willing and apparently ready to do all that. He won today because he took the chances he had to, finishing with 27 winners to Wilander's 13. He made more errors, too, 26 to 16. But the difference between them was more than their margin for error. Lendl was the aggressor. He took the chances and the odds were with him.

He had 10 aces to Wilander's four (and many more service winners); Wilander said that was the difference. Two and a half weeks ago at the ATP championships in Cincinnati, played on the same Decoturf II surface, Wilander beat Lendl, 6-0, 6-3, in the semifinals and McEnroe in the final. His emergence as a hard-court player was heralded.

Wilander had 10 aces against Lendl that day. "In Cincinnati, I played the same but Lendl didn't play as good," Wilander said. "He didn't serve very well and then I could get his second serve into rallies."

Lendl, who noted how much cooler it was today, said, "I made one mistake in Cincinnati and I couldn't get out of it. I tried to pace myself through the match.

"I wanted to start slowly and get into the match. I never really got into the match, getting six points in nine games."

Lendl, the second seed here, got into this match from the start, making it clear that New York is not Cincinnati. When Wilander missed a first serve on the first point, Lendl teed off on the second, a vicious forehand return winner. He was broken before he knew what happened.

Three times in the first set, Lendl held at love. Wilander kept the ball in play but could not generate any pressure until the 10th game.

A backhand error gave Wilander his first break point. Then Lendl established his territorial imperative. Four consecutive service winners gave him the game and the first set.

He made the big serves on the big points. When he squandered a break in the second set, he got mad and got it back. In the seventh game, Lendl anguished as he wasted opportunities (at 15-30 and a break point), swore at himself, collected himself and asserted himself. A forehand pass down the line gave him the second break point of the game. Wilander hit a backhand long for the break.

Lendl held for 5-3 with three aces, one on game point, a definitive statement of purpose.

He broke in the first game of the third set and was broken in the fourth. Disgusted, he bounced the racket off the court, into his hand. "I was a little upset at myself for losing the break," he said. "I didn't want to go for four or five sets because he was starting to play better. I wanted to close it out in three."

The tempo increased, with each of them hitting out, each of them holding his own until the tie breaker. Lendl got the first minibreak to go up, 3-1, with one of his potent inside-out forehands deep into the corner that Wilander meekly hit into the net.

Wilander got the break back as Lendl chose to serve and volley and missed a backhand wide. But with Wilander serving at 3-4, Lendl again demonstrated the versatility Wilander yet lacks, attacking behind a good backhand approach and putting away a forehand volley. An ace gave Lendl match point. A forehand that waslong ended Wilander's day.

Lendl, who lost to Connors in last year's final in four sets, has yet to lose a set this year. "I think I'm the closest one to take a set from him and I'm satisfied with that," Wilander said ruefully.

What about Lendl, Arias was asked. "Let me be happy for one night," he said.