Jose-Luis Clerc was clowning, matching wits with the ultimate tennis cutup, Jimmy Connors. As the two practiced this morning on the stadium court at the U.S. Tennis Center, Connors had called a ball out and Clerc began arguing in jest. He cursed in Spanish. Then he broke up.

And, as he watched, Clerc's coach, Patricio Rodriguez, smiled. Rodriguez firmly believes it is Clerc's new-found ability to have fun on the court that has made him the hottest player in the world during the last two months.

"For a long time Jose never had fun on the court," Rodriguez said today after Clerc went through a brisk hour of practice with Connors in final preparation for Tuesday's start of the U.S. Open. "He has always enjoyed himself off the court, always relaxed whenever he wanted to. But on the court, he was always tense. He could never relax.

"You have to relax to keep up your concentration in a tough match. If you don't relax, you get tired by the end and that's when you lose."

It has been a while since Clerc lost. The 22-year-old Argentine won four straight tournaments on the U.S. clay court circuit this summer, a remarkable feat considering the humidity each week and the length of long rally clay court matches.

Because of that streak, there are some who think Clerc is more than a dark horse here even though he has never been a good fast surface player. John McEnroe, the two-time defending champion, will open play Tuesday as the top seed and favorite, with second-seeded Bjorn Borg, still seeking his first Open title, given the best chance of beating him.

But after that, the field is wide open. Connors has had a poor summer and is seeded fourth, his lowest seeding since 1973. The third seed, Ivan Lendl of Czechoslovakia, had an impressive Davis Cup win over McEnroe here in July, but lost to Clerc twice on the summer circuit.

And then there is Clerc, the fifth seed.

"I feel confident in my game right now," he said today, resplendent in a blue and white Pink Floyd T-shirt. "I am doing many things better, especially my serving. I feel much more comfortable on these courts than I used to."

That was evident this morning as he practiced with Connors. Clerc's first serve is a weapon now, especially on the hard, asphalt courts this tournament is played on. His only noticeable weakness is his second serve, a spinning near pop-up that Connors constantly came in behind.

But Clerc has been getting his first serve in consistently all summer, one of the major reasons for his success. Last year, he came to this tournament after a good summer only to lose in the first round to Bernie Mitton of South Africa. But in his first two appearances in Flushing Meadow, Clerc reached the round of 16 before losing.

Now, just turned 23, he and Rodriguez believe he can win if he keeps his game together. "He is a good enough player now that he can win this tournament," said Rodriguez. "He is much stronger physically than he has ever been and when you have been winning, your confidence just keeps growing. He has always had the power, now he has the consistency, both on his serve and volley.

"The key for him is to remember that this is not clay. On clay, you don't miss many shots. On this surface you have to admit to yourself right away that you will make mistakes and not get down when they happen. If he can do that, he can play with anyone."

Clerc will open Tuesday against Brad Drewitt of Australia. He is seeded to play Lendl, barring upsets, in the quarterfinals with the winner getting McEnroe in the semifinals if the seeding holds up.

That is a far more difficult draw than Clerc has faced all year, although he was impressive in Europe in the spring, winning the Italian Open, then losing a five-set semifinal to Lendl at the French.

"I have played better all the time for the last year," Clerc said."Each week I can see my game improving a little bit more. I think the more I play, the better I will get. I am still excited to play all the time."

During the last three years, as he has risen through the ranks, Clerc has spent a lot of time in the shadow of countryman Guillermo Vilas, a national hero whose every move has been recorded since 1977 when he capped a summer similar to Clerc's in '81 by winning the last Open played at Forest Hills.

Now, with his arrival in the world's top five, Clerc has passed Vilas. He beat Vilas in straight sets twice this summer. He is ranked No. 1 in Argentina -- in the computer, but not in the collective heart of the country.

Vilas, the poet with the long dark hair, looks more like the classic hero than Clerc, whose curly brown hair and high-pitched voice often give him the appearance and sound of a teen-ager.

Indeed, there is a lot of little boy in Clerc, who will, at times, burst into mumbled song during an idle moment, something Vilas never would do. But then, Vilas never had many idle moments; he always was running to another engagement. It is not that way with Clerc yet.