Having watched his team successfully defend the Davis Cup, symbol of international team supremacy in men's tennis, with the most convincing final round victory in the 79-year history of the competition, U.S. captain Tony Trabert today began looking ahead to the 1980 cup campaign with a mixture of optimism and apprehension.

The United States completed a 5-0 domination of Italy in the three-day, best-of-five-match series this afternoon as U.S. Open champion John McEnroe defeated Antonio Zugarelli, 6-4, 6-3, 6-1, and Vitas Gerulaitis trounced Adriano Panatta, 6-1, 6-3, 6-3.

The U.S. thus became the first nation to sweep the final without losing a set, and McEnroe increased his personal Davis Cup singles record to 10-0 without loss of a set. He has won 28 consecutive sets, also a cup record.

The U.S. clinched possession of the celebrated silver punchbowl for the 26th time on Saturday when veterans Stan Smith and Bob Lutz defeated Panatta and Paolo Bertolucci in doubles, 6-4, 12-10, 6-2, for an insurmountable 3-0 lead.

The final singles were a mere formality. McEnroe had beaten Panatta, 6-2, 6-3, 6-4, on Friday evening after Italy's Corrado Barazzutti defaulted with a sprained ankle while trailing Gerulaitis, 2-6, 2-3.

Given this overwhelming superiority and the number of outstanding players he has to call upon -- even if reluctant dragon Jimmy Connors continues to resist his entreaties -- Trabert is justified in having great expectations of retaining the cup next year.

But his caution is equally understandable, since the United States could have to play all its 1980 matches on foreign soil, beginning Feb. 21-23 at Mexico City.

This is an important factor in Davis Cup, not only because noisily partisan home fans can inspire and uplife native players, but also because the host nation supplies the match officials and has choice of court surface and other playing conditions.

For example, the Italian team of Panatta, Barazzutti, Bertolucci and Zugarelli undoubtedly would have been much more formidable on the slow red caly of Rome's Foro Italico, with 9,000 of their screaming countrymen intimidating the visiting players and linesmen alike, than they were on a medium-fast synthetic court at the San Francisco Civic Auditorium.

If Italy were to reach the final again next year, for the fourth time in five years, the U.S. would have to travel to their Roman snakepit, and the results could be quite different.

No one knows this better than Trabert, whose first two years as captain ended in frustrating American defeats at Mexico City and Buenos Aires.

Last year's triumph over Great Britain in Rancho Mirage, Calif., ended a five-year drought for the U.S. In 1974, the Americans embarrassingly failed to get through the American Zone preliminaries, losing in successive seasons to Colombia, Mexico twice, and Argentina.

"It only takes two good players to make a strong Davis Cup team, and the home-court advantage is a big one," notes Trabert, the realist, who remembers the bitter taste of defeat both as a player (the U.S. won the cup only once in his playing tenure, 1951-55) and as captain. "Next year could be a long, tough road trip."

It begins in February, in the 6,000-foot altitude of Chapultepec Park in Mexico City.It was here, on a clay court in front of a raucous crowd, that Raul Ramirez beat Connors in the decisive fifth match the weekend before Christmas in 1975 -- the last time Connors played for the U.S. team. b

If the U.S. beats Mexico, it will play again two weeks late in the American Zone final. That will probably be against Argentina on clay in Buenes Aires, where Guillermo Vilas and Jose Luis Clerc will be much more dangerous than they were indoors in Memphis this year.

Should the U.S. win the American Zone, it would not play again until the third week of September, against the winner of European Zone B. That will most likely be Britain or Czechoslovakia. Either one would have the home court.

If the U.S. reaches the final, the most likely opponents would be Sweden or Italy again. Sweden would have to come to the U.S., but Italy would love to get the Americans in Rome.

For a variety of political, financial and organizational reasons, the U.S.

frequently went into Davis Cup combat in those years without its best players. For example, Harold Solomon and Erik van Dillen were the singles players in the 1974 U.S. defeat at Bogota, while Dick Stockton and Brian Gottfried were the victims in Buenos Aires in 1976.

At least Trabert can be assured of fielding representative teams, which was not always the case in the 1973-77 dry spells.

Now that a sponsor (Congoleum Corporation) pays American players $10,000 per series, plus expenses, Trabert has found it easier to enlist top players.

McEnroe, Gerulaitis, Smith and Lutz cleared their schedules last February, for example, and committed to play all of America's 1979 matches.

McEnroe already has committed himself to the entire 1980 campaign. He is expected to play singles and team with Peter Fleming in doubles. Fleming was disappointed at being left off this year's team after the first series, against Colombia.

McEnroe and Fleming were the best doubles team in the world this year, winning 14 of the 18 tournaments they played together, including Wimbledon and the U.S. Open. "They have earned their shot, and they are my first choice for next year," said Trabert.

Gerulaitis has told Trabert that he will be available against Mexico, if needed, but not for the American Zone finals.

Trabert expects to use McEnroe and Harold Solomon for both matches instead, assuming both are on clay.

"I prefer Solly over Vitas in Mexico City because his topspin shots work well in the high altitude there, and he has an excellent head-to-head record against Ramirez," the U.S. captain said, "I'm trying to get hold of Solly to ask him to keep his schedule open for the first two series, and for the third week of September in case we have to play on clay in Czechoslovakia.

"If we wind up playing Brazil on a faster surface in the U.S. instead of Argentina on clay, of Great Britain on a fast court instead of Czechoslovakia, I'd go for somebody else -- Gerulaitis or possibly Roscoe Tanner."

Nor has Trabert given up totally on Jimmy Conners, who will be No. 2 to McEnroe in the soon-to-be-released U.S. rankings for the year. "I'm planning to write to Jimmy and talk to him. If he wants to play, I'd like to have him on the team," Trabert said.

"I know that a number of players and their agents think a little differently now about Davis Cup than they did a couple of years ago," he continued. "They've seen the plus side of participating and winning, the ensuing good publicity and so forth. Like the press that McEnroe got after last year's final: magazine covers and all that. It didn't hurt his image or his market ability -- and sponsorship helps."

"I think anyone who has the opportunity to play Davis Cup and doesn't is the loser in the long run," he said. "It is a privilege to represent your country, and not many people are given that opportunity. If somebody doesn't want to do it, that's their business. We live in a free society, fortunately. I only wish I know why Jimmy didn't want to play, because he's never said.

"I hope he does play. I always want to put my best team on the court. But we've won it two years in a row now without him, so we know we can do it. Let's face it, if we can figure out how to get by without relying on Iranian oil, we're better off, aren't we?"