This town is one of the few places in the world where a U.S. Davis Cup tennis team led by John McEnroe can be called an underdog.

Argentina produces some of the world's best clay-court players, and its national team is responsible for the only two losses the United States has suffered in the last six years of Davis Cup competition, and two of McEnroe's three defeats in his 39 cup appearances.

"Whenever we go away, we expect rough treatment, because everyone wants to beat the U.S.," said Arthur Ashe, the U.S. captain. "That's normal. But this place is something else."

With the enthusiastic agreement of the avid fans here, Ashe has named Argentina a slight favorite in their opening Davis Cup series starting Friday that both sides are treating like the finals. Because Argentina, playing without Jose-Luis Clerc, lost last year in the cup's first round, the draw has placed two of this year's favorites against each other only four months after the United States handily won the 1982 championships over France.

McEnroe, for one, is not happy about the early showdown. "I think it's wrong," he said. "It's unfair to both teams."

The victory over France was a high point in a somewhat disappointing year for McEnroe, who carried the U.S. team in singles and doubles on his weakest playing surface -- clay. But this playoff is likely to be much tougher for him and the other members of the team -- Peter Fleming, McEnroe's doubles partner, and Gene Mayer.

The competition will begin Friday, with Mayer playing Guillermo Vilas, followed by MeEnroe against Clerc. The doubles will be played Saturday and the reverse singles matches Sunday.

Vilas and Clerc, who will also play doubles, are ranked fourth and fifth in the world by the Association of Tennis Professionals, and they are at then best on clay. Vilas has won the French Open, which is played on clay twice, and Clerc swept all four major U.S. clay tournaments in 1981, the year the U.S. beat Argentina in the Davis finals in Cincinnati. In 1980, both Vilas and Clerc beat McEnroe in Buenos Aires before large and loud Argentine crowds.

"McEnroe is a question on clay," said Ashe. "He's only played two important clay-court matches in two years. We don't know how good he is."

McEnroe will also be playing his first competitive match in three weeks after withdrawing from two tournaments with a slight shoulder injury. But he says the layoff has left him rested, and -- not surprisingly -- he's unwilling to grant Argentina the speculative edge.

"Look, I know these two guys," he said. "I beat them both at the Grand Prix Masters earlier this year. I know they're tougher on clay, and clay is where I'm most vulnerable. But I'm confident."

Even McEnroe's brashness might fade a bit among the clamor of thousands of Argentines who will likely spend the weekend making his concentration as difficult as possible. These fervently nationalistic crowds have been known to demonstrate their displeasure with line calls with showers of the devalued local coin, in addition to intemperate words.

The Argentine team's interpersonal relations, however, tend to be as chaotic as the crowds here. For several years, Clerc and Vilas have carried on a bitter rivalry that has frequently overshadowed the team's efforts, and this year disputes between Vilas and Argentine tennis authorities left his participation in doubt until this week.

By the time the two stars finally arrived in Buenos Aires -- to practice separately -- Argentina's press was near despair. "Until Yesterday, Everything Went Well in the Team for the Davis," one newspaper headline cautiously announced over the weekend. It explained in smaller type, "Clerc arrived and didn't say he wasn't going to play."

The Argentine feuding may help McEnroe and Fleming in the doubles, but it will be the four singles matches -- and, likely, McEnroe's luck with the Argentines on clay -- that will determine the winner.