John McEnroe made a brilliant debut today as a Davis Cup singles player for the United States, but a just as memorable comeback by Buster Motram gave Great Britain a 1-1 split of the first day singles in the best-of-five-match final series for possession of the trophy that symbolizes international team supremacy in tennis.

McEnroe overwhelmed John Lloyd, 6-1, 6-2, 6-2, in the opener of the 67th Davis Cup final, and when Mottram was match point down in the third set against Brian Gottfried, it seemed that the day would be a British disaster.

But long after the sun had disappeared behind the Santa Rosa Mountains and the temperature in this affluent desert retreat outside Palm Springs had dipped to near freezing, Mottram completed a 4-6, 2-6, 10-8, 6-4, 6-3 triumph. The sometimes awful, sometimes inspiring but ultimately gripping match consumed 4 hours, 29 minutes.

Thus, possession of the cup will not be decided until Sunday, when McEnroe plays Mottram and Gottfried opposes Lloyd in the final singles. Americans Stan Smith and Bob Lutz are scheduled to play Britain's David Lloyd (John's brother) and Mark Cox in the pivotal doubles Saturday.

McEnroe's victory was the second most lopsided in a Davis Cup final in the last 30 years.

Only Bjorn Borg's 6-1, 6-3, 6-0 rout of Jiri Hrebec in the opening match of Sweden's 3-2 defeat of Czechoslovakia in the 1975 final at Stockholm was more decisive. Before that, one has to go back to 1948, when Ted Schroeder -- who was in the audience today -- beat William Sidwell, 6-2, 6-1, 6-1, in the second match of a 5-0 American triumph over Australia to find a more one-sided score.

McEnroe, 19, was playing his first Davis Cup singles match -- he partnered Gottfreid to the doubles victory that clinched the American Zone final over Chile at Santiago in September -- but he showed no trace of awe, apprehension or jitters.

"He played like he had come onto the court for a Sunday game before lunch," said Lloyd, who was lavish in his praise of McEnroe after the match.

The 19-year-old left-hander from Douglaston, N.Y., still has a surly deportment, and rabbit ears, though.

Just before tossing up the ball for the first point of the match, he hesitated and numbled a complaint to the umpire. He again started to serve, stopped, and gestured peevishly to the open-air commentary booths above the far end of the court, where American and British television and radio broadcasters were on the air.

"Tell those people up there I can hear every word they're saying," he said.

The umpire asked the commentators for restraint, and the match began. McEnroe served an ace wide past Lloyd's forehand. Then another down the center, past the forehand again.

He missed his next two first serves but held at love, broke Lloyd at love with a backhand down-the-line passing shot and served another ace to start the third game -- a total of nine straight points to start the match.

The third game went to deuce three times, the fourth four times, but the umpire did not intone, "Game, Great Britain" until Lloyd was already 5-0 down.

Lloyd held his serve after seven deuces and four break points in the first game of the second set, held easily for 2-1, but never really got into the match. When McEnroe broke him for 3-2 with two superb backhand cross-court passing shots, the Englishman's spirit sagged and McEnroe was off on another five-game run that took him to 1-0 in the third set.

So dominant was McEnroe that he never even had a break point against him until the second game of the third set. He led 30-15 in that game, but Lloyd clicked off a backhand pass down the line, a nice forehand volley winner and a hard forehand crosscourt return that skimmed the net and forced McEnroe to net a stretching backhand half-volley for the break.

That "revival" was short-lived, however. Lloyd immediately made four erros, mishitting a high forehand volley to lose his serve again at love.

McEnroe kept flying around the court, following every serve to the net for agile volleys, bewildering Lloyd with his speed, anticipation and variety of pace and spin. He kept serving big points to Lloyd's forehand -- "He looked like he expected every ball to the backhand, and kept giving me the forehand, so I took it" -- and ended the match with his sixth ace.

"Nobody ever made me look so much an idiot in my life. He was two classes above me," said Lloyd, a pleasant, earnest 24-year-old who has attracted attention from gossip magazines lately as Chris Evert's latest amour (She watched his play today).

"He must be close to being the best in the world. There's very little difference in class right now between McEnroe, Borg and (Jimmy) Connors.

"I couldn't believe how well he started... I didn't think he could keep it up, but he did," Lloyd continued.

"I didn't have a clue where his serve was going. He kept me under pressure all the time. He didn't give me any time, tearing around at a hundred miles an hour. He was a yard and a half faster than I was. And he seemed to be able to step up his game anytime he wanted.