John McEnroe, who won't celebrate his 20th birthday until Feb. 16, is 14 months too young to drink champagne legally in the state of California, but please excuse him a few illicit sips from the Davis Cup.

In the state of Euphoria, you see, it is customary for members of the victorious team to swig a little celebratory bubbly from the big sterling punchbowl that symbolizes international team supremacy in tennis.

McEnroe, the sometimes temperamental but enormously giffed left-hander from Douglaston, N.Y., clinched the cup for the United States today and completed the most extraordinary singles debut in its 78-year history by dazzling Englishman Buster Mottram, 6-2, 6-2, 6-1, in an exquisite 92-minute display of quickness, athleticism and racket control.

McEnroe's total domination on the cement stadium court at Mission Hills Country Club -- he lost only 15 points in 12 service games and never had a break point against him -- gave the U.S. an unbeatable 3-1 lead over Great Britain in the three-day, best-of-five-match final series and possession of the Davis Cup for the first time since 1972.

That rendered the final singles academic, and Brian Gottfried cruised to a pressureless 6-1, 6-2, 6-4 victory over Britain's John Lloyed to make the final score 4-1, United States.

The U.S. has now won the cup, donated by Dwight Davis of St. Louis in 1900 as the "International Lawn Tennis Challenge Trophy," a record 25 times.

Australia, last year's winner but dethroned by Great Britain in the semifinals in October, has been victorious 24 times. Britain, which reached the final for the first time since relinquishing the cup to the U.S. in 1937, has won nine times; France, six times; Sweden, South Africa and Italy, one each.

McEnroe -- who whipped John Lloyd 6-1, 6-2, 6-2, in Friday's opener -- teamed with Gottfried for the decisive doubles point in the American Zone final at Chile in September, but had not played a cup singles match until this weekend.

He left no doubt about his mettle in pressure matches, though, roaring like a raging sandstorm through this affluent desert community outside Palm Springs -- an unlikely and unsatisfactory venus for such a grand old event.

Never in 67 previous Davis Cup finals had a player lost so few as 10 games in two singles matches.

The previous most one-sided performances were by Bjorn Borg in Sweden's 3-2 victory over Czechoslovakia in 1975, by Jack Kramer in a 4-1 U.S. victory over Australia in 1947 and by Bill Tilden in a 5-0 American triumph over Australasia (Australia and New Zealand) in 1924. Each yielded 12 games in two matches.

In 1904, when Great Britain beat Belgium, 5-0, in the first year that more than two countries entered the competition, Laurie Doherty won his first match, 6-4, 6-1, 6-1, and his second by default. Fifty-five countries played Davis Cup this season.

But so much for ancient history. Let us talk of McEnroe, whose storied past, one assumes, is in front of him.

So complete was his mastery today that Mottram, 23, never got more than two points against his serve in any game.After one uncharacteristic sloppy patch in which he made five consecutive errors -- "I guess I got lazy for a few seconds and took my eye off the ball," he said -- McEnroe won 10 of the last 11 games, 49 of the last 59 points.

"On form, he's playing like the No. 1 player in the world right now," British captain Paul Hutchins said of McEnroe, who has won four tournaments since the U.S. Open in September, and last month beat the reigning No. 1, Borg, for the first time, in the semifinals of the Stockholm Open. That was the first time Borg, 22, had ever lost to a younger player and astonishingly, he won only seven points on McEnroe's serve in the match.

Mottram is no Borg, but he is a Davis Cup fighter. He proved that Friday, keeping underdog Britain alive by resurrecting himself from two sets and a match point down in the third to beat Gottfried, 4-6, 2-6, 10-8, 6-4, 6-3, in a 4 1/2-hour epic.

That gave hope to the 100 flag-waving, vocal British supporters who came 6,000 miles on two package tours to forget about British reserve and cheer like fury for their nation's first cup triumph in 42 years.

Their cries of "Come on Buster, do it again!" rang out in the crowd of perhaps 4,000 -- attired in shirtsleeves and sun dresses instead of the heavy sweaters and coats of the first two days -- on an idyllic 70-degree desert morn.

But McEnroe did not take long to demonstrate that he was as radiant as the Sunday sun.

In the fourth game he got to 15-40 on Mottram's serve with a buzzing forehand down the line that forced a lunging forehand volley error. On the next poing, he flicked a magnificent backhand lob, practically on the baseline, over Mottram's head.

The gangly, plodding, 6-foot-4 Mottram turned and made a futile effort to reach the ball, tripping over his feet and tumbling ignominiously to the cement.

Hutchins, neutral referee Pancho Contreras, and a couple of linesman rushed to his aid, but McEnroe just shrugged, looking quite self-satisfied, like a prizefighter who had knocked his foe down and adjourned gleefully to a neutral corner.

That was the beginning of a decisive TKO.

McEnroe served superbly, intelligently, mixing up the pace, spin and placement of his deliveries. Time and again he jerked Mottram so far out of position on wide serves, to both forehand and backhand, that the Englishman had no hope of putting his racket on McEnroe's first volley to the other side of the court.

McEnroe did not so much beat Mottram as dissect him, opening up the court with his quickness and command of a variety of spins, then cutting Mottram's heart out with volleys as sharp as a scalpel.

"It's a waste of time going in!" an anguished. frustrated Mottram shouted to Hutchins in the second game of the second set. He tried staying back on second serves, and some first serves, but that was an ineffective tactic on a fast, hard court. McEnroe beat him from the back court and thrashed him from the net.

"Too fast" screamed Mottram in the seventh game of the third set, after failing to get within six feet of a killing McEnroe volley. It wasn't evident if he meant the court surface or his opponent. Probably both.

McEnroe, who had six aces, got 41 of 63 first serves in the court, Mottram 47 of 64. But many of McEnroe's second balls were better -- deeper, with more devilish spin and placemeant -- than Mottram's first serves.

McEnroe also attacked Mottram's second serves, getting in behind deep, forcing returns.

"I saw him stay back against Brian the other day, so I decided I'd hit the return hard and come right in off it," McEnroe said. "He started staying back more, which wasn't very smart because it gave me a chance to take the net the whole match."

McEnroe is a brash, flip, and quickwitted youngster. He does not have much savoir faire. He came to the cup presentation wearing his powder boue team blazer over jeans and an openneck shirt, instead of the team uniform-gray slacks and navy Davis Cup tie -- worn by veterans Stan Smith and Bob Lutz, who won the pivotal doubles point over David Lloyd (John's brother) and Mark Cox yesterday. But he can play the game.

"I still marvel at his racket control," said Tony Trabert, who was ecstatic at his first triumph after two disastrous seasons as the U.S. captain. (The U.S. had not gotten through the American Zone in four years.).

"I'm proud of all the players who helped us get here -- not only the four who played in the final, but Harold Solomon, Arthur Ashe, Vitas Gerulaitis, Sherwood Stewart and Freddie NcNair, who helped us get this far. We've been down the other road the last couple of years -- we didn't win -- and so this is a great thrill for me personally and for the players.

"I was particularly pleased with John's reaction to having won," added Trabert, referring to McEnroe's thrusting his arms in the air and jubilantly batting a ball out of the stadium after the final point. "He gives you that blase routine, but he was pretty excited."

Underage or not, he deserved champagne. CAPTION:

%picture 1, U.S. player John McEnroe reaches for a low shot from Buster Mottram in Davis Cup match. AP; Picture 2, McEnroe waves an American flag after his convincing win to clinch U.S. victory. UPI