They call him "Batata," or sweet potato.

"Because when I am young and a little bit fat, I cannot move so well on the court," said Jose-Luis Clerc.

In the last year, not much has gone right on the court for Clerc. His ranking dropped from fifth to 10th. He failed to get past the second round in nine of 10 tournaments. After losing to Fernando Luna in the second round of the French Open, he was so depressed he considered quitting. "I was feeling so bad, I said maybe I've got to stop," said Clerc, 24. "One day, I said to my coach, 'I cannot play any more.' "

Clerc could smile now. "He said, 'Wait a minute.' "

Then, Monday night, he won the U.S. Pro Tennis Championship in Boston, his first win since January. He did not lose a set all week and beat Jimmy Arias, 6-3, 6-1, in the final.

He arrived in Washington today. "Batata is back," he said.

Clerc, top-seeded in the D.C. National Bank Classic, will play Eddie Dibbs in a second-round match tonight. Dibbs, playing with an arthritic left hip, beat Jimmy Brown, 3-6, 6-2, 6-1, last night, outslugging and outmaneuvering his 18-year-old opponent.

"They start so young," Dibbs said, sounding aged at 32. "All these guys can hit a million balls. I had to beat Brown to play Clerc. Clerc and me played in finals a lot of times."

It was another soggy, sodden day of tennis at Rock Creek Stadium and another seed succumbed to it. Eric Fromm, who is ranked 47th in the world and seeded 12th, lost, 7-5, 6-3, to Bruce Foxworth, who is ranked 189th. Pablo Arraya, the No. 9 seed, defeated Alejandro Ganzabel, 6-1, 6-4.

Brown lost to Clerc in the quarterfinals in Boston last week, but won six games--more than anyone else all week. "He's playing a lot better," Brown said. "I think he lost interest a little bit. . . I think he lost the drive to win."

Last year, when he came to Washington, Clerc was the defending champion and had won two tournaments in a row. Not quite the streak of 1981, when he won four consecutive tournaments and 27 straight matches in one blistering hot summer, but not bad. Tired, he struggled here last year, and lost to Arias in the semifinals.

Perhaps the problems began then. Who can say what makes that certainty of self suddenly so elusive?

In January, he signed a new racket contract and abandoned his standard-sized wooden Rossignol for a midsize Kennex. For tennis players, who are often as high-strung as their rackets, this is no small decision.

Suddenly, he could no longer feel the ball, could not guide it. He won the Hollywood Classic in Guaruja, Brazil, with it, but lost in the first round of the next tournament he played. In March, he beat John McEnroe at home on clay, 2-6, 10-8, 6-1, 3-6, 6-1, to help Argentina beat the United States in Davis Cup. "I was happy because I beat McEnroe," he said, "not because I played well."

He lost in the first round in the next two tournaments to Shlomo Glickstein and to Bjorn Borg. It was Borg's last tournament: in the locker room in Monte Carlo, he was relaxed, joking. Clerc was tense, expected to win. He lost badly, 6-1, 6-3. His confidence, already vanishing, disappeared. "Every time I tried to hit it, it went into the crowd," Clerc said.

"Nobody knew how Borg was playing," said Clerc's coach, Patricio Rodriguez. "Everybody was going, 'Borg, Borg.' Nobody even looked at him. It could have started it."

Clerck took some time off, but it didn't help. Three more tournaments, three times he failed to get past the second round. He seemed to be playing better at the Italian Open, got as far as the quarters. But one night, upset by line calls, he walked off the court during a doubles match, not realizing that it meant he was out of the tournament in the singles as well.

When he lost to Luna in the second round of the French Open, 6-2, 6-1, 6-1, on the red clay at Roland Garros--so much like the clay of Argentina--Clerc hit bottom. "For him, it's like McEnroe losing in the first round of Wimbledon," said Rodriguez.

Clerc said he was "pretty close" to calling it quits. "He told me. He was pretty dramatic about it," Rodriguez said. "There was no way in a tournament like that that he could lose to Luna . . .

"He was serious about it. We didn't know what was happening. I thought something with control of the ball, feel of the ball, wasn't there."

So after Wimbledon, where he lost in the first round again, they went to Kennex and talked racket. The company developed a prototype standard-size racket, which he used for the first time last week in Boston. "You got to see the difference," he said. "I feel midsize is not for my game. It's hard to guide the ball."

Rodriguez said he had never seen Clerc play better than he did in Boston. Rodriguez watched as Clerc practiced on a court outside his hotel room, complaining facetiously about a bad call, smiling the whole time. "You can see the difference today, just practicing to have fun," Rodriguez said. "You can tell he's back. He's back to life."