Like the young Rod Laver, the red-haired Australian who began captivating tennis audiences two decades ago, John McEnroe at age 19 is both acrobat and magician with a racket in his left hand.

He does not hit the ball as hard as "the Rocket," but his athleticism and racket control, his quickness and command of spin, and his flamboyant, Laver-like backhand enable him to improvise shots no one can recall having seen before.

And his serve is better than Laver's was -- "the best I've seen in a long time," says colleague Eddie Dibbs. Menacing, clevely varied, it is a serve that in the best lefty tradition slithers unpredictably.

But unlike the taciturn and undemonstrative Laver, to whom his tennis is increasingly being compared, John Patrick McEnroe Jr. has a volcarnci personality.

A child of the rebellious '60s, an adolescent of the "me-first" '70s, and a New Yorker to boot, he is bright, brash, sassy, fhppant. The flip side of Rod Laver.

McEnroe made his first major impression on the tennis world as the Wimbledon wunderkind of 1977, at 18 the youngest semifinalist in the 100-year history of the sport's oldest tournament. He came across as a precocious brat -- immensely talented, spolied and rather obnxious.

On court, he pouted, cursed, threw his racket, ranced at line umpires and generally engaged in unbecoming conduct. He was a crybaby.

Off court, suddenly and unexpectedly thrust into the limelight, he demonstrated little savoir faire . Scoffed one appalled gentleman after encountering a sticky-fingered McEnroe in the players' tea room: "The boy wonder is upstairs, eating the traditional strawberries and cream without benefit of the traditional spoon."

Although he is smart enough to have known better -- "He got Bs at Standford without studying; you have to be bright to do that," says Arthur Ashe -- McEnroe did little to dispel this unflattering image the next year If anything, he magnified it, temporarily eclipsing Jimmy Connors as tennis' enfant terrible .

In his first year of playing with the big boys -- full time in the summer of 1977, part time during his freshman year at Stanford -- McEnroe frequently behaved like a juvenile.

He argued line calls, fumed and moaned incessantly. If he hit a spectacular winner, he looked as if he expected nothing less. If his opponent hit one, he looked unhappy. If McEnroe missed a shot, he looked unhappier.

It was this Last Angry Man demeanor, the succession of snarls that crossed his "Irish pug face," that alienated officials and spectators, suggested Curry Kirkpatrick in a Sports Illustrated profile. They brough to mind "the eternal anughty child who never got enough of the back side of a hairbrush."

Like all perfectionists, McEnroe was never satisfied with his performance. He expected more of himself, and so his expressions seemed to say that tennis was torture, not fun.

It is difficult to change such an image. Once branded as a graceless punk, it is not easy to become a hero. But McEnoe is trying.

In the last four months -- during which he has won five tournaments in singles, eight in doubles, and spearheaded the first U.S. victory in the Davis Cup finals since 1972 -- he has improved his behavior as dramatically as his tennis.

"I still find him difficult to like, but it's no longer impossible not to hate him," said more than one viewer who watched McEnroe come back from 1-4 in the final set and save two match points in beating Ashe in the recent Colgate Grand Prix Masters final on national TV.

For a youngster who has heard more jeers in a year than most tennis players do in a lifetime, that was a great leap forward.

"He is at heart a good kid. I firmly believe that," said Ashe, who has spent a good deal of time with McEnroe this fall and winter. ("We talk all the time. Or I should say I listen all the time, because he never stops talking.")

"It's unreasonable to expect a 19-year-old, soon-to-be-millionaire, New York-oriented, flashy athlete to behave like he's 30 years old," Ashe went on. "He has done some immature things, but he's not incorrigible. In fact, I'd say he has grown up pretty damned fast."

Ashe was turned off, as were many patrons, by McEnroe's puerile behavior last March at the Volvo Classic in Washingotn. But he says now, not unreasonably, "We all judged the kid too soon."

"You've got to realize how pampered American's elite junior players are today," Ashe said recently. "When I was coming up, discipline was enforced with an iron hand. We were told how to act, and we obeyed. Now the attitude is: 'Want to throw your racket? Fine. Throw it. Want to slam a ball and cuss? Go right ahead. Just don't lose this match!'

"The emphasis is all on winning because the rewards are enormous -- a chande to be a millionaire before you're 21, to be on the cover of News-week and Peopel, to be surrounded 360 degrees by television cameras when you beat Jimmy Connors.

"So what are you going to do ? Juniors come to the pro game spoiled to death, and then we at the last minute try to say, 'Hey, we love the way you hit the ball and pull in pectators, but we're professionals up here and we have certain standards by which we operate. Please don't hurl your racket because you might decapitate somebody, and we'd appreciate it if you didn't swear too loud and give the finger. We know you're not used to this, but if you want to play with us, you'll have to abide by our rules."

McEnroe often has been characterized as a street kid, but that is wholly inaccurate.

He grew up in the fashionable suburb of Douglaston, 30 minutes by train out on Long Island from Manhattan. His father is a successful attorney, specializing in corporate law for a Wall street firm and now manages his son's business affairs. At home, John Jr. is still treated as just another member of the family, but none of the McEnroe kids make their own beds.

John went to Trinity High, a demanding parochial school in Manhattan, where he played basketball two years and was an All-Sta left wing on a good soccer team. He skipped his graduation exercises to play tennis in Europe for the first time, won the French Junior title and then became the first player ever to come through the qualifying preliminaries and reach the semifinals at Wimbledon.

Suddenly a celebrity, welcomed as a "wild card" to any tournament he cared to play that summer, McEnroe was tempted to turn pro and play full time. At his parents' urging, he enrolled at Standford as previously planned.

By the time McEnroe who the NCAA singles title as a freshman last June, and promptly turned pro, he was ready.

Having spent his first 18 months in the public eye creating scenes and getting people mad at him, he has since refocused attention on his talent: the dazzling net play, remarkable half-volleys, Laver-like topspin backhand and variety of stroke. And especially that sneaky southpaw serve that spins more wildly than a crooked roulette wheel and cuts like a street fighter's blade.

"He never overpowers anyone, but he has a ton of shots," Ashe has said. "It's slice here, nick there, cut over here. The wounds aren't deep, but pretty soon you bleed to death."

Junior, as McEnroe is known on the tour, has become Mac the Knife.

Despite losing in the first round at Wimbledon to Erik van Dillen -- a disappointing thud after the previous year's heady coming-out party -- he had a good summer.

By gaining the semifinals of the U.S. Open in September, he became the first male since Ken Rosewall in the 1950s to have reached the semis of both Wimbledon and the U.S. championships while still a teen-ager.

And since Connors best him at the Open, McEnroe has compiled a spectacular record, winning 49 of 56 singles matches, Grand Prix tournaments at Hartford, San Francisco, Stockholm and London, his first two matches as a Davis Cup singles player, and the Masters. With Peter Fleming, he won both the WCT World Invitational Doubles in London and the Masters. In six months as a pro, he has collected $463,866 in prize money.

At Stockholm, on a lightning-fast tile court, McEnroe beat Bjorn Borg for the first time, 6-3, 6-4.

In his Davis Cup singles debut at Rancho Mirage, Calif., McEnroe lost his serve only once in trouncing John Lloyd, 6-1, 6-2, 6-2, and Buster Mottram, 6-2, 6-2, 6-1. Never in 67 previous cup finals had a player lost as few as 10 games in two singles matches.

More than any other sport, tennis is notoriously capricious in posting the hypothetical question of who is No. 1, and answering with the name of the hotshot of the moment. Thus, it was perhaps inevitable that Ashe's pre-Masters statement that "McEnroe has been the best player in the world since September, no question about it," was misinterpreted to mean he had already usurped Borg's throne.

Borg was unquestionably the No. 1 player of 1978, a notion reinforced by his designation last week as "world champion" by the International Tennis Federation. Connors was No. 2. McEnroe is moving up rapidly in the fast line -- he may already have passed Guillermo Vilas and Vitas Gerulaitis to become No. 3 -- but he understands that the people who would proclaim him king are premature.

"I've got to prove it first," he said at the Davis Cup, and reiterated at the Masters, after scoring his first victory in five meetings with Connors.

They played point for 10 games until Connors aggravated a blood blister on his foot. Trailing, 5-7, 0-3 and limping badly, Connors quit -- unwilling to give McEnroe the satisfaction of a victory without an asterisk on it. He later conceded nothing, saying that McEnroe "played pretty good," but that he had not improved since the Open. He implied that the truncated match meant nothing.

McEnroe, sensibly, had another view.

"I think a victory over Connors is a victory over Connors, and I won that match," he said pointedly of the TKO. "I would rather have won, 7-5, 6-0, but beating him, I'll take it any way I can get."

Psychologically, he said, the result will certainly help him the next time they meet, possibly this weekend in the U.S. Pro Indoors at Philadelphia.

But McEnroe pooh-poohs those who rush to call him No 1.

"I didn't beat Connors all during the time this fall that people said I was maybe the best. Now I beat him, but he quit, so it's still not a victory in most peopel's minds," he said during the Masters.

"And I beat Borg on a surface he doesn't like, so you have people downplaying that a little bit. You have to beat Borg on clay in Paris, or beat Connors on hard courts at Flushing Meadow, before people are convinced. That's what it comes down to."

Some people however, are convinced already.

"He hasn't won any of the Big Four titles yet, but he will before it's over," says Ashe.

"I think he's a tremendous plus for the pro game because he's young, American, daring, tremendously talented, and his game depends more on flair, a la Nastase, than it does on mechanical repetition, like a Chrissie Evert or a John Newcombe.

"I used to love to watch Nastase play. Now I love to watch McEnroe play. The guy hits some incredible shots. There aren't many players who can do what he does with a tennis racket, and that's exciting."

Ashe used to love to watch Rod Laver play, too.