John McEnroe's tennis, stumbling and sporadic for two long days on this town's unfamiliar clay, finally fell flat against the steady back court game of Guillermo Vilas today and the United States was eliminated by Argentina in the first round of the 1983 Davis Cup.

McEnroe needed a victory over Vilas to keep faint U.S. hopes alive in the best-of-five match competition. Instead, he offered a limp shadow of his usual Davis Cup form and suffered his worst defeat in six seasons of tough competition, 6-4, 6-0, 6-1.

The loss rounded out a weekend in which the U.S. team, the defending champion, never adjusted to the clay courts of the Buenos Aires Lawn Tennis Club, the unrelentingly noisy capacity crowd or the polished defensive games of Vilas and Jose-Luis Clerc. It was the third straight time a U.S. team had lost in Buenos Aires.

"We just got outplayed here," said Arthur Ashe, the U.S. Davis team captain. "These are two of the best clay court players in the world."

The U.S. strategy in the match depended all along on McEnroe, who needed to beat both Vilas and Clerc in singles and win with Peter Fleming in doubles to ensure a victory. The second American singles player, Gene Mayer, has historically been outplayed by Vilas and Clerc; he lost to Vilas in straight sets Friday.

McEnroe was evidently not ready for the challenge. Following a slight shoulder injury, he took three weeks vacation before coming to Buenos Aires. He had played in only two important clay court tournaments in the previous 1 1/2 years.

After losing a five-set marathon with Clerc Friday and Saturday morning, then narrowly winning with Fleming in doubles after another five sets Saturday afternoon, McEnroe was clearly worn out. He was uncharacteristically quiet and appeared almost listless. After the match, he left immediately, skipping a scheduled press conference and turning down requests for interviews.

This afternoon, with the playoffs already decided by McEnroe's loss, Clerc withdrew from the second singles round in favor of 23-year-old Alejandro Ganzabal. Mayer defeated him, 6-4, 3-6, 10-8, making the final score of the round 3-2 for Argentina.

"You cannot judge a player by the way he plays in Davis Cup," Vilas said of McEnroe. "There are a lot of demands that aren't there in a tournament. McEnroe didn't have enough strength to overcome two back court players at once."

And the U.S. team, lacking top singles players outside of McEnroe, does not have the strength to win a Cup match without him. "It would have been nice," Ashe said quietly after today's match, "if we had had our absolutely best team with us."

That was largely a reference to Jimmy Conners, who has consistently refused to play Davis Cup matches in recent years. Conners announced this year he would never again compete on clay--thus indirectly rejecting Ashe's invitation to play in Buenos Aires.

The United States will play Ireland late this year to hold its position in the Cup's championship bracket.

Ashe said he will ask Conners to play against Ireland. "I'll never stop asking him," he said. "I think there's a chance he will decide to play."

Argentina's best previous performance in the Davis Cup came when it lost to the United States in the 1981 finals. It will play Italy in the quarterfinals and will be a strong favorite to reach the finals if Vilas and Clerc play.

For Vilas, today's match became a relatively smooth exercise of using his most comfortable game to wear down a tired McEnroe. "I played correctly," he said. "I played the right tactics. The prime ingredient was to move (McEnroe) around constantly."

McEnroe's only opportunities came early. Vilas started sluggishly, and McEnroe broke his serve in four straight points in the first game. Vilas offered McEnroe three chances for a second break in this next two service games, but McEnroe could not break open the set, losing the last two break points on unforced errors.

Then, with McEnroe serving at 4-3, Vilas slipped into his groove. McEnroe netted two volleys and a ground stroke, then pushed a forehand long and was broken at four points. Vilas won his next serve easily and went on to break McEnroe's serve seven times in a row.

The rout was so complete that even the crowd quieted somewhat, as if abashed by Vilas' control. McEnroe showed almost none of his usual agressiveness, neither in playing or in his court manner. In the last two sets, he said nothing, made few visible gestures and spent much of his time between points slowly walking in circles with his head down.

When he finally won his serve at 0-5 in the third set, avoiding a second love-set loss, McEnroe raised his arm in ironic triumph and the crowd quietly applauded.

As in his match against Clerc, McEnroe was unable to find the range for his first serve. He missed more than two-thirds of them--and was often forced into long back court rallies that are Vilas' specialty. When he tried rushing the net on his return of serve, he was driven back with passing shots that Vilas consistently placed along the lines.