Arthur Ashe debuts as U.S. Davis Cup captain this weekend when the United States plays Mexico at the La Costa resort in Carlsbad, Calif. With John McEnroe and Roscoe Tanner playing singles, and veterans Marty Riessen and Sherwood Stewart primed for the doubles, the United States should have no problems against a Mexican team made up, as Ashe put it, of "Raul Ramirez and three guys you've never heard of."

The Davis Cup competition has been streamlined this year. Sixteen nations battle for the trophy that symbolizes international team supremacy in men's tennis, and 46 other countries engage in a second-level competition in four geographic zones for places in the main draw next year.

Prize money has been injected for the first time: a million-dollar purse, including $200,000 for the cup-winning team. Gone are the days when players donated their services for flag and glory. U.S. players are being paid $15,000 per series, plus what they win. "If we go all the way and somebody plays in all four rounds, he could make more than $100,000," says Ashe.

The U.S., with by far the deepest pool of talent, is always favored, but Ashe knows the road can be deceptively bumpy. The U.S. has fallen into potholes six of the last eight years, five times losing early to teams from south of the border: Argentina (1977 and 1980), Mexico (1975-76) and Colombia (1974).

Mexico's undermanned team -- the experienced Ramirez and youthful cup newcomers, Jorge Lozano, Juan Hernandez and Adolfo Gonzalez -- wouldn't seem to have a prayer on the medium-fast cement court that so well suits McEnroe and Tanner, with their oppressive left-handed serves. But La Costa is near the Mexican border, and the visitors will have vocal support in the best-of-five-match series (McEnroe versus Lozano and Tanner versus Ramirez today, doubles Saturday, Tanner versus Lozano and McEnroe versus Ramirez on Sunday).

"The place is sold out -- 5,400 tickets each day, and at least 1,500 or 2,000 of those people will be screaming for Mexico," Ashe said yesterday. "Davis Cup is different. Funny things can happen."

He knows. He played Davis Cup for 10 years, winning 27 singles matches, more than any other American player in history. But in 1967, he lost both his singles to guys nobody had ever heard of. Miguel Olvera and Pancho Guzman, as a two-man team from tiny Ecuador, beat the U.S., 3-2, at Guayaquil, perhaps the biggest upset in 81 years of Davis Cup play.

"That was the absolute nadir of my career. No tennis event ever depressed me so much. I felt I had let my teammates, my country, my captain and myself down," recalls Ashe.

He lost the decisive match to Guzman, a defensive clay-court road runner, by the curious scores of 0-6, 6-4, 7-5, 0-6, 6-3. Riessen shared in the ignominy, choking badly in the pivotal doubles.

It was a painful episode all around. The elated Ecuadorian captain tried to leap over the net to embrace Guzman, but tripped and broke his ankle. U.S. captain George MacCall was so distraught he smashed his fist against a steel door in the locker room, and went to pieces emotionally.

The Ecuadorians outside the madhouse arena, who were unable to get in and join the "Adios, gringos" celebration, tossed rocks over the walls. One missile just missed U.S. player Cliff Richey, who nearly precipitated an international incident by screaming at officials to "stop those savages."

Such are the passions and pressures of Davis Cup.

Of course, La Costa is not Guayaquil, or Bogota, or Mexico City. This time the Americans are not playing on the hated slow read clay, as they were in those cities and in Buenos Aires last year, when Argentinians Guillermo Vilas and Jose-Luis Clerc won all four singles over McEnroe and Brian Gottfried.

Ramirez, ranked only No. 36 in the world, is seldom the inspired player he was in lifting Mexico past the U.S. in 1975 at Palm Springs and 1976 at Mexico City, where he beat Jimmy Connors in the decisive fifth match.

Ashe says his men are ready. McEnroe likes the Davis Cup; he led the U.S. to victory in 1978 and 1979, and had never lost a set until the double-ambush in Argentina last year. Tanner is playing well again after a dreadful 1980 season.

Connors, whose reluctance to play Davis Cup over the years contributed to his unpopularity was at La Costa this week as a sparring partner. Ashe expcets him to join the team for an anticipated second-round showdown against defending champion Czechoslovakia in July.

"Provided," he says with proper deference to the strange things that can happen in Davis Cup, "Czechoslovakia beats Switzerland and we beat Mexico."