It took two long sets for John McEnroe to find his game here today on the outdoor clay, and that gap may have been more than he could afford in the United States' Davis Cup match against an inspired Argentina.

Argentina led the United States, 1-0, in the five-match competition after today's play, and was one game away from a second point when McEnroe's match with Jose Luis Clerc was called by darkness with Clerc leading, 6-4, 6-0, 3-6, 4-6, 5-2. The match will be finished Saturday.

"Our backs sure are against the wall," said Arthur Ashe, the U.S. team captain, after the suspension of play. Following a 6-3, 6-3, 6-4 victory by Guillermo Vilas over Gene Mayer in today's first singles match, Clerc could give Argentina a commanding advantage if he serves out a victory against McEnroe.

Ashe can only hope that he gave McEnroe a strategic boost by asking for play to be called after 4 1/2 hours tonight. The suspension leaves McEnroe due to serve, down by one service break, "and if he can win his serve, I think Clerc could be made very nervous serving for the match," Ashe said. "If John breaks his serve, he'll really have momentum."

On that slim margin rests much of the United States' hope to win its first Davis Cup match in Buenos Aires after losses in 1977 and 1980, the only defeats the United States has taken in the last six years of Cup play. U.S. players have consistently stumbled on the unfamiliar clay courts of the Buenos Aires Lawn Tennis Club before large, glamorous crowd--and today was no exception.

Mayer, locked into a slow defensive game and plagued by errors, offered little resistance to Vilas. It was Mayer's fourth loss in four matches against the Argentine. And for the first two hours of the afternoon match, it appeared that McEnroe would never begin to play.

"Clerc played about average," said Ashe of Argentina's No. 2 player, fifth in the world. "McEnroe was just missing it. The story is confidence on clay--it took John two sets to get it, and by then we were two sets down."

Only one of McEnroe's familiar gutsy comebacks prevented his worst defeat in six seasons of Davis Cup competition. Clerc, serving powerfully and clearly inspired by the crowd, dominated the pace as McEnroe struggled and fumbled with his control. Before McEnroe recovered, Clerc had handed him his first love-set loss in Cup competition and led, 6-4, 6-0.

Uncharacteristically cautious in the early game, McEnroe often remained on the base line after his serve, only gradually shifting to the serve-and-volley attack that is his trademark. The style led him into costly unforced errors on ground strokes and his first serve failed him again and again.

He gave Clerc the first service break in the match when, serving at 2-2, he double-faulted to drop behind, 15-30, then double-faulted again at break point two points later. McEnroe then had a chance to break Clerc's serve in the next game, but netted an easy forehand at break point. Clerc served out the game and went on to win the set easily with strong serves, ending with an ace.

The second set brought the near complete collapse of McEnroe's serve. As a capacity crowd of about 9,000 panted, whistled and blew horns with every miss, McEnroe placed only six first serves in during the set and double-faulted on one of the three break points that Clerc swept.

When McEnroe's form finally came, it came in classic style. As the second set ended, McEnroe began to sharply question line calls by the Italian judging team. During the third set, he stopped play three times to question calls, returned some of the catcalls that poured down on him from the crowd, and pounded one ball cross court after missing a short volley--leading to a one-point penalty.

"Just don't throw anything that hits me," McEnroe growled at one point as the crowd briefly whistled and honked at his complaints and missed shots.

Meanwhile, he regained his serve and broke Clerc's serve in four straight points in the fourth game. Then, he grittily held on during five break points in his last service game to win the set, 6-3. The fourth set followed a similar pattern as McEnroe won, 6-4, with another single break over Clerc.

"I've seen him come back a lot of times like this," said Ashe. "Once he got going, he didn't have much trouble."

Clerc, however, rallied as the fifth set began and, when McEnroe netted two volleys and double-faulted in his first service game, Clerc made the break that leaves him in position to win when play resumes.

Following the end of McEnroe's match with Clerc, McEnroe and Fleming are scheduled to play doubles against Vilas and Clerc in the second day's competition.