Guillermo Vilas further buried his old reputation as "The External Second" today, beating defending champion Jimmy Connors in a thrilling finale to the U.S. Open tennis championships, 2-6, 6-3, 7-6, 6-0.

Vilas came back from a 1-4 deficit in the third set, and two set points at 5-6, to turn the match around in a tie breaker that he won, 7 points to 4.

The victory was Vilas' 39th in a row, his 46th consecutive on clay courts. It gave him his seventh consecutive tournament victory and his second Grand Slam title this year. He started his clay-court streak by winning the French Open.

The match started at 4 p.m. - the time dictated by CBS-TV, which also had change overs extended from one to two minutes to accommodate commercials - under a bright blue sky dotted by billowy clouds that disappeared as the shadows lengthened into early evening.

It was a gorgeous early fall day, with a football nip in the air, and a tricky wind that swirled in the stadium of the West Side Tennis Club, changing directions and velocity.

"I'd say Vilas' chances went way up when it got windy," suggested Arthur Ashe. "He hits with so much more spin, it's easier to keep the ball in on a windy day. Jimmy won't get a clean crack at the ball; it will be darting all over the place."

But Connors was controlling his hard, flat strokes masterfully in the first set, ripping them deep, low over the net, ignoring the low margin of error he affords himself.

The first four games went on serve, both players slugging the ball deep in torrid backcourt rallies. No genteel pitty-patting or moon balls here. This was trench warfare, with all guns firing.

But Vilas lost his serve at 30 on a double fault in the fifth game, after saving two break points from 0-40.

Connors held from 0-40 in the next game, saving four break points. He was off and running on a five-game splurge for the set.

At the end of that first set, Connors looked in complete command. He was dictating the blistering pace of the rallies, and Vilas could not slug with him.

But at in this year's Wimbledon final against Bjorn Borg, Connors had his man on the ropes early in the second set and couldn't deliver a knockout.

He had Vilas at 0:30 in the first game, after a double fault, but knocked a backhand down the line wide after having a setup off a net cord. Then he made four consecutive forehand errors, foreshadowing a reversal.

Connors had Vilas at 0-40 in the third game, and the bull-like Argentinian lefthander won five straight points to get out of it.

The tide was turning. Connor's forehand approach shot, especially on short balls, was starting to desert him. That is always the most vulnerable part of his game, and though Vilas was slow in starting to attack it, he eventually did.

Connors was also having increasing trouble at the south end of the court, where the wind was most treacherous. And Vilas was getting used to the savage pace of his fellow lefthander's service returns.

Vilas broke at 15 for a 5-3 lead in the second set, Connors hitting backhands wide on the first and last points.

Vilas served out the set at 15. The same spectators in the sellout crowd of nearly 13,000 who had been whispering Vilas' obituary sensed his expanding confidence.

The match turned in Vilas' favor when he erased a 1-4 deficit in the third set and won a tie breaker, 7 points to 4.

Connors broke after two deuces for a 2-0 lead, after the lights had been turned on at 5:30. Vilas saved three break points in that game, but Connors waded in and slashed away a forehand cross-court volley.

He held for 4-1, but lost his serve in the seventh game after leading 40-15 - one point from a 5-2 lead.

From 40-15, Vilas hit a backhand winner, Connors netted a backhand, and Vilas laced first a forehand and then a backhand passing shot, both down the line.

Vilas saved three break points in the next game, holding from 15-40 when Connors netted a forehand approach after three deuces.

Vilas hold from 15-40 - two set points - when he served at 4-5. He poured an ace down the middle, past the forehand, forced a volley error with a buzzing backhand cross-court pass, and put in another good serve that Connors returned long off the forehand.

As in so many crucial situations, Vilas got himself out of trouble with big serves to the forehand.

Vilas broke at love for a 6-5 lead, but was broken straight back at love as he served for the set. It had indeed become a war.

The tie breaker was the turning point, just as the third set tie breaker that Connors won from Borg in the final here last year was in that match.

From 3 points all, Vilas hit a winning forehand volley and forced a forehand error by a good return of Connors' serve. Connors get to 4-5 with a forehand volley, but then Connors missed a net cord forehead and Vilas blasted a forehand down-the line pass off a forehand cross court.

Losing that tie breaker finished Connors. Vilas held with four straight points from 0-30 in the first game and roared through the final set, gaining momentum as he approached his second major title of the year.

Connors saved three match points from 0-40 in the last game, but when Connors steered a forehand wide - a late call - on the last match point, the crowd spilled onto the court to embrace the jubilant Vilas.

Connors and Vilas, both 25, surprisingly had met only twice before.

Their first encounter was in the final of the Western Championships at Cincinnati in 1972, before either had climbed into the upper echelon of the game internationally. Connors won, 6-3, 6-3.

They did not meet again until the semifinals of last year's U.S. Open here. Connors demolished Vilas in that one, 6-4, 6-2, 6-1, and went on to beat Wimbledon champ bjorn Borg in the final, 6-4, 3-6, 7-6, 6-4, to reassert his grasp on the No. 1 world ranking he first earned in 1974.

To get to the final this year, Connors thumped Jasjit Singh, 6-2, 6-0; Bob Lutz, 6-2, 6-2; Zan Guerry, 6-1, 6-4; No. 11 seed Roscoe Tanner, 6-0, 6-2; No. 5 seed Manuel Orantes, 6-2, 6-4, 6-3, and Corrado Barazzutti, 7-5, 6-3, 7-5.

Vilas, similarly, did not lose a set in reaching the final hero for the first time.He defeated Manuel Santana, 6-1, 6-0; Gene Mayer, 6-3, 6-0; Victor Amaya, 6-3, 6-3; Jose Higueras, 6-3, 6-1; Ray Moore, 6-1, 6-1, 6-0, and No. 12 seed Harold Solomon, 6-2, 7-6, 6-2.

Vilas had lost only four sets in winning his previous 38 matches in a row, only five sets in racking up 45 straight matches on clay courts.

Coming into the open, Vilas had won his last six tournaments - at Kitzbuchol, Austria; Washington; Louisville; South Orange, N.J.; Columbus, Ohio, and Rye, N.Y. The clay court streak also included his triumph in the French Open, his first Grand Slam title.

However, he had not met either of his rivals for the unofficial No. 1 world ranking - Connors or Borg - during the streak. They were not entered in any of the tournaments Vilas won.

Connors, having a slightly disappointing year by his own lofty standards, came to Forest Hills with four victories in 14 tournaments. He was runner-up in six others, including Wimbledon, where he lost to Borg in five sets. His biggest victory was over Dick Stockton in the World Championship of Tennis final at Dallas in May.

Betty Stove, a semifinal loser to champion Chris Evert in the women's singles, and South African Frew McMillan, who did not get a game from Ilie Nastase in the first round of the men's singles, each captured a second doubles title today.

Stove and Martina Navratilova defeated Renee Richards and Bettyann Stuart in the women's doubles final, 6-1, 7-6. Stove then teamed with McMillian - who had taken the men's doubles for the first time with Bob Hewitt on Friday - to beat Billie Jean King and Vitas Gerulaitis, 6-2, 3-6, 6-3 in the mixed doubles final.