How much difference can an empty chair make in the outcome of a tennis match?

That will be a question in the back of people's minds this weekend as an undermanned but emotional Italian team attempts to wrest the Davis Cup, symbol of international team supremacy in men's tennis, away from a superior U.S. squad that includes two of the world's top four pplayers.

The vacant seat at the courtside is the Italians' way of honoring the memory of Umberto (Bitti) Bergamo, their captain who died in an auto accident in October, five days after Italy beat Czechoslovakia, 4-1, to reach the final round of the Davis Cup for the third time in four years.

Bergamo, 49, was driving from Florence to a tournment in Barcelona when his sports car slammed into a tractor-trailer truck at high speed. He had been close to his players -- especially to Adriano Panatta, the national heartthrob who was instrumental in having him appointed captain last year. When word of his death reached Spain, Panatta wept. "I have lost the greatest friend I ever had in tennis," he said.

Panetta, a player of rich talent but unpredictable mood, will face John McEnroe in the second match of the three-day, best-of-five-match series Friday night. Corrado Barazzutti, whom the Italians call "Soldatino" ("Little Soldier"), will play Vitas Gerulaitis in opening singles at the San Francisco Civic Auditorium (WETA-TV-26, 10:30 p.m.).

Bitti Bergamo's chair will be there, unoccupied, a constant reminder of the friend to whom the Italians have dedicated this final. It could inspire them.

Panatta is ranked only 27th in the world, according to the computer of the Association of Tennis Professionals. But he has a rare combination of power and touch, and when his heart and mind are in a match, he can play with anyone. The 1976 Italian and French Open champion usually plays well in Davis Cup, and if he decides to "win one for Bitti," McEnroe could have his hands full.

Panatta is the key to the Italians' chances. He will team with the similarly gifted but streaky Paolo Bertolucci against veterans Stan Smith and Bob Lutz in the doubles Saturday, and will play Gerulaitis in the final singles after Barazzutti plays McEnroe Sunday.

Panatta's tennis can range from the sublime to the ridiculous, but only if he is at his best will the visitors have a real chance to capture the gleaming silver punchbowl they have won once before, in 1976.

This is a longshot, admittedly.

The American team is undeniably stronger. McEnore is ranked No. 3 on the computer, but in the opinion of most knowledgeable observers is the second-best player in the world right now, behind only Sweden's Bjorn Borg.Gerulaitis is No. 4, behind Jimmy Connors. Barazzutti, who has had a bad year, is ranked only 32nd.

But strange things have happened in the splendid history of the cup that Harvardian Dwight Filley Davis launched in 1900 as the "International Lawn Tennis Challenge Trophy."

The Davis Cup is always filled with passion, more so than any other tennis event. Even supremely confident players have gotten "goose skin," as the Italians call it, when called upon to represent their countries. Emotion is an important element, and Bergamo's empty chair an intangible factor.

Americans, accustomed to outstanding athletes in so many sports, tend to be blase about the Davis Cup. Few natives seem to know or care that the final is taking place here this week, the U.S. defending the cup it has won 25 times previously.

But just how much the "Coppa Davis" means in Italy is illustrated by the fact that 37 Italian journalists have come here to chronicle the final.

The matches will be televised live to Italy, starting at 2 a.m. Saturday morning. More than 5 million viewers are expected to stay up and watch through the night, until it is time for croissants and coffee.

Even though all five matches will be televised at home, a tour group of 500 Romans arrived here Tuesday and will be cheering in person, waving red, green and white Italian flags.

The Italian press corps, a critical lot, gives their side no chance to win.

"We are like lambs to the slaughter, but everybody comes, anyway, because San Francisco is such a nice town," said one newsman from Milan.

The American players are too good, they say. McEnroe, the U.S. Open champion, is 8-0 in Davis Cup singles, never having lost a set. Gerulaitis, the Italian Open champ, is 8-2. Both have winning records against Panatta and Barazzutti.

The surface, a medium-fast indoor carpet, favors the home team, they say. McEnroe and Gerulaitis are at their best on it. Panatta and, particularly, Barazzutti prefer a slower court. "Little Soldier" has been known to surrender indoors.

The Italians' hottest player at the moment -- Gianni Ocleppo, a serve-and-volleyer who plays best indoors -- is not even on the four-man playing team. He is along solely as a practice partner, they say.

"This is a scandal," declared Alfonso Fumerola of Rome, one of the 37 Italian journalists. "Panatta, Barazzutti, Bertolucci and (Antonio) Zugarelli are a Mafia. When Bergamo became captain, he had to agree to this team. Ocleppo is beating everyone now, but he is not selected. It is a scandal, I tell you, but even with Ocleppo we have no chance."

Tony Trabert, who played Davis Cup for the United States in 1951-55 and is now in his fourth year as non-playing captain, listens to all this and smiles.

He knows he has better talent. He knows that McEnore and Gerulaitis are serious enough about Davis Cup that they passed up a tournament in Montreal last week in which Borg won $180,000 -- McEnroe to rest, Gerulaitis to tend to a strained hamstring in his left leg that he now says is "100 percent."

But he also knows that this is Davis Cup, so he is wary. "Funny things can happen, with the emotion and all," he said.

He knows that the Italians have been in San Francisco since Dec. 4, practicing like crazy. He knows they are serious too -- so much so that Vittorio Crotta, who replaced Bergamo as captain, asked to have the balls weighed, to make sure that they are not lighter and therefore faster than agreed upon, which would give the U.S. a further advantage.