Veterans Bob Lutz and Stan Smith, probably representing the United States for the last time in a Davis Cup final, defeated Italy's Paolo Bertolucci and Adriano Panetta, 6-4, 12-10, 6-2, today to clinch America's 26th possession of the trophy symbolizing international team supremacy in men's tennis.

The doubles victory -- which hinged on a tense, 85-minute second set that ended with Bertolucci losing his serve for the first time -- gave the U.S. an unbeatable 3-0 lead in the three-day, best-of-five-match series, and reduced Sunday's final singles match to a formality.

U.S. Open champion John McEnroe will play Antonio Zugarelli, a substitute for the injured Corrado Barazzutti, and Vitas Gerulaitis will oppose Panatta in the concluding singles, with only the final margin of the U.S. victory at stake.

In Friday's opening singles, McEnroe played superbly to wallop Panatta, 6-2, 6-3, 6-4, after Barazzutti severely sprained his right ankle and had to default to Gerulaitis while trailing, 2-6, 2-3. McEnroe has not lost a set in nine Davis Cup singles matches over two years, and has now won 25 consecutive sets, more than any previous American in the 79-year-history of the cup.

But this afternoon, as McEnroe and Gerulaitis watched from the gallery at the San Francisco Civic Auditorium, the day belonged to sentimental favorite Lutz, 32, and Smith, who turned 33 Friday.

The former University of Southern California teammates first played Davis Cup doubles together in 1968, and they clinched the decisive point in U.S. victories in 1968-69-70.

Over 12 years, they compiled a Davis Cup record of 11-1, the only defeat coming at the hands of Australians Phil Dent and John Alexander in wind-blown Sydney during America's 4-1 semifinal victory in October.

Today, they knew that they were likely celebrating the last hurrah of a distinguised tour of duty for the U.S. team.

After reigning as America's top-ranked doubles pair for most of the last decade, they have been supplanted by McEnroe and Peter Fleming, who this year have won 14 of the 18 tournaments they have played together, including Wimbledon and the U.S. Open (won, fittingly, over Smith-Lutz).

U.S. captain Tony Trabert stuck with his veterans this year, despite some criticism, because he made a commitment to them last February, when he asked them to clear their schedules for the entire cup campaign. This was before McEnroe and Fleming had emerged so clearly as the No. 1 tandem in the world.

Trabert already has informed all parties involved that McEnroe and Fleming are his first choice for doubles in 1980. Lutz and Smith understand this. They have accepted it, although Smith, in particular, dreams of making a triumphant return in the future.

"This may have been our last time. We're not sure. But when 'The Star-Spangled Banner' was playing, I was just thinking to myself that I wanted to make it one last good performance," said Lutz, who did just that. He was the only player who did not lose his serve in the match, and he kept constant pressure on the Italians with solid returns of serve from the left court.

"I enjoy playing Davis Cup, and I hope to be back," said Smith, who also grasped the cup-winning point in singles in 1971-72.

"After you've been playing the circuit for a few years, the average tournaments all blend together. Davis Cup -- which is the most prestigious event in which you can represent your country in tennis -- and the major tournaments stand out. It's a great challenge. There's a lot of pressure when you play. I really get excited about it.

"I don't think this was necessarily our last match, but I knew it could be, so I wanted to play well."

Smith and Lutz were not nearly as overpowering as they were in routing Mark Cox and David Lloyd for the go-ahead point in America's 4-1 final round triumph over Great Britain last year, breaking a five-year drought for the U.S.

They were playing a more formidable team this time. Panatta-Bertolucci came into the match with a 14-4 record in Davis Cup doubles, and Bertolucci played an inspired match in the right court.

"I think Bertolucci was the strong man today. He made some shots I had never seen before," said Smith, explaining the midmatch shift of their primary target to the dashing, 6-foot-2 Panatta from the stubby, 5-7 Bertolucci, a gifted but rather lazy player who answers to the nickname "Pasta Kid."

"They both have very strong forehands, especially on the second or third shots, so we tried to stay away from that side. But once we got into the match and saw how well Bertolucci was playing, we tried to make Panatta hit the first volleys."

The Americans lost their serve only once in the match. Bertolucci hit a diving backhand that rolled along the net cord and dribbled into the alley for a winner to break Smith in the seventh game, after Panatta had lost his serve on a double fault in the previous game.

But Panatta lost his serve again for the set, with Lutz doing most of the damage. After Panatta double-faulted to 30-40, Lutz floated a forehand return that Panatta pushed long with a low forehand volley, his second volleying error of the game on that side. He slammed down his racket in disgust.

No tie-breakers are used in Davis Cup, the most traditional of tennis events, and the pivotal second set turned into a marathon filled with nervous tension.

The Italians in a crowd of 5,398 spectators sang choruses to each other during change games, waved miniature red, white and green Italian flags, chanted "Italia, Italia," and noisily expressed their displeasure in word and gesture on the several occasions that Panatta and Bertolucci disputed calls.

"On some points, I really couldn't tell if we were in Rome or San Francisco, but that's to be expected,' said Lutz, who was the most consistent player on the court. He returned, volleyed and served solidly while Smith was more streaky, contributing numerous spectacular shots and deft lobs but returning poorly in stretches, especially off the backhand.

The Italians saved a set point in the 16th game, Bertolucci slashing away a backhand first volley on Panatta's serve, but they could never get more than two points on the Americans' serve.

Finally, in the 22nd game, the Americans broke Bertolucci from 40-15 -- the first time in eight service games that they got more than two points against him.

Italy had one advantage point, but Smith ripped a backhand volley that split the defense. Then the Americans got the advantage when Bertolucci netted an uncharacteristically careless forehand first volley. Both Panatta and Bertolucci argued and gesticulated passionately that the serve had ticked the net, but to no avail. They had argued so many calls by then that there was little sympathy for them.

On the break point, Panatta tried to put away a backhand volley, Smith reflexed it back, and Panatta hit another killer backhand from close in. Instead, he hit the ball in the net.

After that, the heart went out of the Italians quickly. Panatta lost his serve in the sixth game of the final set, pushing a forehand volley long off a smith backhand return and a backhand half-volley wide off a nicely angled backhand return by Lutz.

Bertolucci then lost his serve for the match, Lutz coaxing a perfect topspin winner into the corner on the final point. That was an appropriate ending, a triumphant flourish for a grand old team making its probable Davis Cup swan song.