Vitas Gerulaitis defeated Roscoe Tanner with a magnificent comeback from two sets and a service break down and John McEnroe routed injury-inhibited Jimmy Connors today to set up a hometown duel between Long Island neighbors in the men's singles final of the U.S. Open tennis championships.

Gerulaitis, denied any foothold in the match until he seemed out of it, courageously dug in and climbed back to beat Wimbledon runner-up Tanner, 3-6, 2-6, 7-6, 6-3, 6-3, in a memorable three-hour match.

That was a thrilling triumph for the 25-year-old extrovert from King's Point, but he was upstaged as late afternoon turned to twilight by McEnroe, his 20-year-old neighbor from Douglaston, N.Y., who thumped defending champion Connors, 6-3, 6-3, 7-5.

Connors -- the champion of 1974-76-78, who failed to reach the final for the first time in six years -- had been treated for severe lower back strain and spasms Friday afternoon. A ferocious competitor, Connors battled to the end -- coming back from 1-5 to 3-5, 15-40 on McEnroe's serve in the first set and breaking McEnroe when he served for the match at 5-4 in the third -- but he was obviously restricted by the pain in his back the last two sets.

"I don't think I played great. I played solid," said McEnroe, who also played cleverly. If he can repeat his mastery of Gerulaitis in their two meetings this year -- 6-0, 6-3 indoors in Milan and 6-3, 6-3 three weeks ago in the Canadian Open, on an asphalt-based hard court similar to that used for the Open -- he will become the youngest U.S. champion since Pancho Gonzalez in 1949.

The two Long Islanders, both first- time finalists in one of the world's three top tournaments, will play for a $39,000 first prize and the most prestigious title in American tennis on Sunday (WDVM-TV-9, 4 p.m.) at the National Tennis Center, less than 10 minutes from their homes.

This will be the first All-American men's final since Tony Trabert -- now a CBS-TV color commentator -- defeated Vic Seixas in 1953, but ironically neither player has been able to captivate the hometown fans.

"That's the ultimate irony," said the gifted but dour McEnroe, who like Gerulaitis has been chagrined by the lack of support from New York spectators. "They won't know who not to cheer for."

McEnroe, who seems destined to challenge Bjorn Borg for the world No. 1 ranking before too long, served and volleyed well and smartly took pace off the ball in beating Connors for the third time in five meetings this year. But he was also aided by Connors' injury.

Connors' -- who was also beaten in the semifinals of the French Open this year by Victor Pecci and in the semis of Wimbledon by Borg -- left without talking to reporters after his first loss since becoming a father Aug. 1.

But Dr. Irving Glick, the tournament physician, said he had treated Connors Friday for "a real acute back strain and muscle spasms -- really severe."

Glick said Connors "could hardly bend over" when he arrived at the trainer's room Friday afternoon after running into a fence while practicing on the court at Gerulaitis' house. He was treated with ethyl chloride, anti-inflammatory medicine, lutrasound and massage, and was given a back brace.

"He was fine this morning when I examined him. He had no complaints. It was surprising he responded so quickly and so well, considering the severity of the spasms -- but he is a gutsy guy," said Glick.

He was fine at the start of the match, but these things often reoccur, especially when somebody does as much twisting and lunging as Jimmy."

McEnroe, rushing the net on his serve and keeping the ball in play with paceless shots that induced reported unforced errors from Connors, raced to a 5-1 lead before Connors showed any lack of mobility.

Connors got one service break back, and when he ripped two forehand return winners to get to 15-40 as McEnroe served at 5-3, he reacted with all his old swagger. He arched his back, thrust his arms in the air, pumped his fist and marched to the deuce court in a high-stepping strut. He was arrogance personified -- his adrenalin pumping, the veins in his neck bulging.

But then Connors was late on a backhand return, knocking it wide and McEnroe clicked off a backhand down-the-line pass, a forehand first-volley winner and a decisive ace down the center to snuff out the threat and seal the set.

Connors was never the same thereafter,although he did taunt McEnroe after his temperamental fellow left-hander screamed petulantly at the umpire in the fourth game of the second set.

Connors had lost his serve in the first game, double-faulting to 30-40 and netting a backhand approach, and he lost is again at love on three unforced errors in the night game.

It was in that set-ending game that Connors failed to chase a McEnroe drop volley, leading to speculation about his fitness. After losing his serve at 15 on another unforced error in the second game of the third set, Connors started gingerly touching his back. At change games, he was stretching, his nose practically touching the court as he tried to loosen up, and he occasionally grimaced and grabbed at his back.

Even when he broke McEnroe with a backhand lob winner as he served for the match at 5-4 -- after a double fault to 15-40 -- Connors showed no fight, no theatrics. He seemed lifeless, subdued -- either resigned to losing or afraid of jerky movements.

"He had an off day, no doubt about it," said McEnroe after breaking Connors' serve for the match with a forehand approach and forehand crosscourt volley. "He usually makes quite a few unforced errors, but he also hits a hell of a lot more winners than he did today."

Asked if he thought Connors was injured, McEnroe said sarcastically: "Did I see it? I didn't notice it. I'm surprised I didn't win by default." He had won two earlier matches, over John Lloyd and Eddie Dibbs, by default.

Gerulaitis, a flamboyant shotmaker who intimidates opponents with his speed afoot and agility at the net, seemed hopelessly out of today's match when he trailed by two sets and dropped his serve at 15 in the first game of the third, double- faulting twice (one on a foot fault) and netting a volley.

As the ball bounced back at him off the net, he angrily bashed it at the linesman who had called three foot faults on him, but it sailed into the stands and hit one of the 18,213 spectators gathered around the stadium court. Apparently heading for a crushing disappointment, he couldn't even get mad right.

"If I could just have held my serve for 2-0 in the third set, I think he would have been pretty demoralized. That had to be the turning point," said Tanner, who later second-guessed umpire Frank Hammond for not penalizing Gerulaitis a point for batting the ball at an official and into the stands.

Instead Tanner -- who had played brilliantly to eliminate top-seed Borg on Thursday night -- played his first bad service game, double-faulting at 15-40.

Thereafter, he seemed to forget until he was too far behind in the fifth set all the things that have made him such an improved player this year. He stopped moving his feet. He became erratic and almost indifferent on the service returns and passing shots he had been hitting so punishingly. He stopped putting pressure on Gerulaitis' serve. From 0-1 in the third set, Gerulaitis lost only nine points in his next 11 service games and only one point on his serve in the pivotal tie breaker that he won, 7 point to 5.

Tanner had been whacking his top-spin backhand -- the main technical improvement in his game at age 27 -- and playing aggressively on his own serve, following every one to the net and volleying firmly.

But suddenly, his supreme confidence evaporated. He started missing his first serve too often and sailing his ground strokes. Gerulaitis volleyed more to his forehand, and it crumbed.Frustrated, Tanner tried to muscle his first serve when in trouble, and his accuracy plummeted to a mere 45 percent for the match.

Gerulaitis, now 4-1 in his career rivalry with Tanner, had said beforehand the key would be hanging in long enough for Tanner's serve to go off, like a fast ball pitcher losing his stuff. He was right, and in the third set he started playing the pace of Tanner's deliveries, blocking them back instead of swinging at them. Many of his counterpunches fell in for winners.

As Tanner's first serve vaporized, Gerulaitis took advantage of his shortening second serves, crunching returns and getting to the net behind them or shipping to Tanner's feel and teeing off for passing shots on the weak half-volleys or low volleys that came in reply.