They are so alike and so different. Yannick Noah is tall, smoky, emotive. A Frenchman. Mats Wilander is compact, blond, reserved. A Swede.

They are clay-court players by experience and inclination. Noah attacks, Wilander revels in his base line authority. Both announced their arrival in the upper echelons of tennis on the clay of Roland Garros by winning the French Open. Wilander won it a year ago. Noah beat him in the final this year.

Today, they arrived at the U.S. Open to take on the hostile hard courts of the National Tennis Center. Although they won first-round matches, both said it was premature to talk of winning the championship. "I'm not ready to win the U.S. Open yet," said Wilander after beating Guy Forget, 3-6, 6-1, 6-2, 6-2. "Maybe in a few years."

"I'm not ready, either," Noah said, after beating Scott Davis, 6-1, 3-6, 7-6 (7-2), 6-4. "But if I play match point in the final, I'll try to win it."

So, Noah was asked, will he and Wilander be serious threats in this tournament? "No," he said, "we're just going to be on vacation."

It seemed stranger coming from Wilander. After all, he beat John McEnroe and Ivan Lendl on the same surface two weeks ago, when he won the ATP Championship in Cincinnati. The tournament seemed to herald his arrival as a force to be reckoned with on something other than clay.

Noah's self-effacement was easier to understand. Today's match, which was interrupted by rain for 77 minutes with Davis leading, 1-0, in the fifth, was only his third since the end of his 42-day suspension by the Men's International Professional Tennis Council for failing to appear at a World Team Cup match last spring.

What's more, he has been suffering from tendinitis in his left knee, a problem off and on for the last year. Although he iced it during the match, he said the knee did not hurt today. "I'm scared about it," he said. "I don't want to hurt it more."

Their matches (and perhaps the entire tournament) represent a step in the development of complete players. And so, they were the most interesting of the day. The other veterans of the surface toiled easily on it. Chris Evert Lloyd, the defending women's champion and six-time Open winner, lost her serve in the first game of her match but won the next 12 games, beating Shelly Walpole, 6-1, 6-0, in 52 minutes.

"I'm not the greatest starter, anyway," she said. Just one of the best finishers.

Ivan Lendl, the No. 2 men's seed, who lost to Jimmy Connors in the final last year, seemed more at ease with his surroundings, and more at ease with the give-and-take in the press room.

"I'm never looking for tough matches. If you give me 6-2, 6-0, 6-2, I'll take it until the end of the tournament," he said.

That was the score by which he defeated Florin Segarceanu. Lendl never lost his serve or his sense of humor. When asked whether he would appeal his suspension by the Czech Tennis Federation for his participation in an exhibition in Sun City, he smiled and said, "It doesn't work that way in Czechoslovakia. Suspension there is for real."

Once he might have snarled at the inquiry.

Ilie Nastase, who has done his share of snarling in his time, served 26 aces but lost to Peter Fleming, who is best known as McEnroe's doubles partner, 7-6 (7-3), 6-4, 2-6, 2-6, 7-6 (7-3).

Pam Shriver of Lutherville, Md., the fifth seed, needed only 43 minutes to dispatch Laura DuPont, 6-0, 6-3. Shriver eliminated Martina Navratilova last year in the quarterfinals. Navratilova, the No. 1 women's seed, did not play because of the rains that came and went and delayed the afternoon schedule.

The skies, heavy and clouded, threatened all day. Drizzle briefly interrupted Wilander's match in the first set with him trailing, 1-4. The rain was making the fast court even faster and less to his liking.

Forget, a young Frenchman with a marvelous first serve and strong ground strokes, had Wilander on the defensive, serving five aces in the first set.

It just reinforced Wilander's feelings about the surface. "I like clay more because you have more time," he said. "Today in the first few games, there was nothing I could do to stop his serve. It was like he was doing 18 holes in one all the time.

"On clay, you can slow down and hit slice. Here it is aces all the time and I must be fast as a flash to reach the ball."

But Forget, who is ranked 127th, could not maintain the pace. His backhands became more errant. His forehand down the line went wide more often than it hugged the line. Wilander, who is the personification of patience, waited for his openings and took them when they came his way. He broke at love in the fourth game of the second set and ran off a streak of seven straight games, breaking Forget the first time he served in the third set.

Forget fell behind, 0-40, and beseeched himself, "Allez, allez." He saved two break points, but then found himself where he didn't want to be: in a long base line exchange with Wilander. Those he could not win. Sensing this, he became impatient and went for a backhand that found the net.

Wilander, who is 19 and seeded fifth, said, "I am definitely not in the top three on this surface. I need to work on my first serve and volley and come to the net more.

"Maybe I will never get there. I never liked playing hard courts and I had to change my attitude to play on them. Clay court will always be my favorite surface and I will always play most clay-court tournaments. But I will work to be better on grass and hard courts."

Which he must do if he ever hopes to be No. 1. "I don't care if I am ever No. 1," he said. "I just want to be better in tennis . . . Everyone can't be No. 1."

In this way, too, he resembles Noah, who is not sure being No. 1 is as lofty a goal as others might think. This, also, may be a stage in development, a defense against self-doubt. Noah certainly went into the match unsure how he would play after such a long layoff (he lost in the third round to Fleming in Montreal in his first tournament after the suspension and in the first round of an exhibition last week to Johan Kriek).

"You are not match-tough," he said. "You need confidence from playing. My practice was not very impressive today but my play during the match surprised me. I moved much better than I thought."

Davis, a Californian who won the national 18 and under hard court championship in 1979, was at home on the surface. "They are very fast," Noah said. "The surface is different from last year. I prefer slower courts."

Davis served and volleyed and made Noah uneasy in the match that was delayed by rain for 77 minutes at the beginning of the fourth set. But Noah, the fourth seed, gradually began to assert himself. In the third-set tie breaker, he took a 2-1 lead with a forehand return at Davis' feet; he won the next three points, one of them with a daring, reflexive backhand swat--a block, really--of an overhead. His athleticism had won the day.

Later, Noah was asked if winning the French Open had helped give him confidence today. There was a faraway look in his eye. "It was so long and far away," he said, a reminder of how ethereal a victory can be.