Illie Nastase pranced into the players' dining room at Stade Roland Garros, holding hands with his nephew, Ion-10-year-old son of Ilie's older brother Constantin, a former player who is now coach and captain of the Romanian Davis Cup team.

The man all the tennis world knows as "Nasty" fussed over the lad, played games with him, showed him off to his colleagues. "He will be a very good player," said uncle Ilie. "He just won the Romanian championshiop of age 8 to 10."

Nastase has a way with children perhaps because he is a child in so many ways. Antoine de Saint Exupery wrote, in "The Little Prince," that all adults were children once, but most of them don't remember it. Nastase, 30, has never acted grown-up long enough to forget.

Off the court, when he's playing pranks, joking and needling everybody he knows, amusing the children of tennis-playing parents, Nastase is irresistibly charming.

An infectious joie de vivre lights up his handsome, delicate Latin features and the mischief that twinkles in the dark eyes that change shade like a mood ring is all playful. At such times he is the gentlest, kindest of human beings, an absolute deligth-"the good Ilie," as his wife Dominique describes this side of him.

"The bad Illie," who disputes calls, cons opponents, swears and makes obscene gestures, infuriates even his most partial fans, and disrupts tennis matches with singularly vulgar and tempestuous scenes, is only too well known to the sports commmunity.

The catalog of nasty incidents has grown so long, the boorishness so familar, that it is all rather tiresome. "The bad Ilie" is more famous than the good because that is the public eye he is forever spitting in.

In his first three matches in the French open, there were no memorable Nastase eruptions, only mild rumbles and grumbles, and an occasional finger to the crowd and rude comment to an official. "The Balkan Peace Sign," as he calls his favorite gesture.

But with Nastase, a destructive storm can develop without warning. Says Davis Cup teammate Ion Tiriac, a former friend adn mentor, "Nastase is like a little dog that you spend a long time training, teaching tricks and social graces. Then, just when you think he will make you proud, instead he makes a puddle in the middle of the floor."

Despite the lighthearted moments with his nephew and others, the flashes of "The good Ilie" that come so naturally to him, Nastase has not been in a good mood here. The enigmatic Romanian, who is blessed with all the gifts of genius and plagued by all its eccentricities, is tired. The world is too much with him.

On the court he has been unimpressive. He was listless in his first-round victory over Lito Alvarez, inconsistent and sometimes downright sluggish in beating Tim Gullikson and Karl Meiler, both in five sets.

He is the No. 1 seed, favored to win his first grand slam title since he reigned here in 1973, but neither his heart nor his mind seems to be in it.

And because nastase is so much a player of mood, of inspiration, knowlegeable observers doubt he will win unless his spirits improve.

Nastase was more resolute today in beating Jan Kodes, 6-4, 7-5, 6-3, to advance to the quarterfinals. He made few extraordinary shots, but played consistently good and resourceful tennis. There was little niggling, only his usual assortment of pained and neurotic glances.

This was Nastase's second straight-set victory over Kodes in a month. Three weeks ago he beat him easily in Rumania's Davis Cup victory over Czechoslovakia.

Nastase is the subject of a personality piece in the June issue of "Playboy." The article is entitled "Ilie the Terrible," but it recounts tales of the good Ilie as well as the bad and the ugly. On balance, it is a sympathetic look at this incurable Peck's Bad Boy of tennis. It gives the devil his say.

Nastase, who is sensitive to critism, read the Playboy article. "This one was not bad, not too good," he said. "There are some things that shouldn't be in there."

Later he explains he was unhappy with some of the unpleasant language attributed to him. He did not dispute the veracity of the accounts, but he wishes they were not in print. "But if story is in Playboy," he said, "doesn't matter, you can have everything."

Nastase is sitting alone now in the dining room, fiddling nervously with the strings of his racket. His Belgian wife is chatting with friends a few tables away.

An acquaintance happens by and asks Nastase about his daugher, Nathalie. "She is 2 years old now, big like this," Ilie says, raising his arm four feet above the floor. "We are trying to have another child. Maybe next year." He pauses. "I would like to be home now, making babies."

Home for Nastase is Bazoches Sur Betz, France, 70 miles from Paris, in the Picturesque Loiret region. "Is very nice, very beautiful," he says longingly. "Is green, with castles all around."

He knows it will be a long time before he gets home, however. After the French, he is scheduled to play in two grass-court tournaments and at Wimbledon, then join the Los Angeles Strings of World Team Tennis for the second half of their season. Nikki and Nathalie are going with him, but the road schedule is heavy, and Ilie also must return to Romania for Davis Cup play.

His inlaws have a country home at Bazoches (Nikki's father is a well-to-do banker in Brussels, Illie's a retired bank cashier). Nastase has a country estate. His parents are living there now, their home having been destroyed by the earthquake that devastated Bucharest earlier this year.

"It was unbelievable," Ilie said, suddenly pensive and nostaglic, thinking back to the quake. "I was in the U.S. I tried to find out if my family was O.K. I heard nothing for four days. I was going crazy.

"Then they contacted me, they said everything is fine. Is not until I go home for Davis Cup that I see the house is destroyed. But all right. Houses you can have. At least they were not hurt.

"I had a house in Bucharest, too, a split level I built a couple of years ago. Now I have half a house. The roof, half the rooms are gone. Every house in the city was damaged, I believe."

As he sat strumming his racket, Nastase changed moods half a dozen times in half an hour. He is mercurial by nature and seems especially so now.

His sense of humor can be basic, but he also can be hysterically funny. Unlike Jimmy Connors, who has long tried to emulate him but lacks the style, Nastase is a fine natural comedian.

But this day the clown had more sad-faced moments than usual. He keeps saying how fatigued he is. He would like to give up the pursuit of hundreds of thousands of dollars, the stylish clothes, cars and all the trappings of success. This most Westernized and capitalistic of East Europeans wants to go home.

"I've been playing Davis Cup already 11 years," he says. "Can you believe it? But now I just don't feel like playing tennis.

"I think the most enjoyable time for me in tennis was when I was very young," he said. "I didn't make any money, I just enjoyed to play. I played for hours and hours every day, and I loved it.

"When there was no money around, I didn't feel any pressure. When you start to win lots of money, is different story. Before I was content with little things-little food, little money, little anything. I was very happy. Now, not so much.

"I'm not tired physically, but in the mind. I don't feel like practicing today, but everybody else does it, so I have to practice. I would like to take a few weeks off and be ready for Wimbledon, but every week I am in tournaments.

I'm not enjoying to play for the moment because I play four months with no stops. Most of the time indoors. I don't like so much. When you are not enjoying is difficult to play well, but this happens to everybody sometimes.

"Last year I played (World) Team Tennis, then I was eager to play tournaments again. Maybe that is the difference. If I went home, I would play tennis maybe with my wife, but would be different."

He stalled a while, then frowned when he saw two Americans heading for a practice court. He got up and motioned to his wife that he was going outside.

"Look at all those guys practicing," he growled. "I don't want to, but I have to. Is like a rule."

Ilie Nastase, who never liked playing by the rules, got up and went out to practice.