Bjorn Borg hammered Jimmy Connors in the semifinals of the Wimbledon tennis championships today even more decisively than he did in last year's startling one-sided final. Now, only first-time finalist Roscoe Tanner stands between Borg and his fourth consecutive singles title.

The astounding 23-year-old Swede dazzled the crowd of 15,000 spectators jammed around the Centre Court of the All England Lawn Tennis and Croquet Club on a brilliantly sunny day, ripping shots of remarkable pace and accuracy as he flogged Connors, 6-2, 6-3, 6-2, in 1 hour and 49 minutes.

Compared with that extraordinary demonstration of tennis, the second semifinal in which Tanner beat fellow Stanford grad Pat DuPre, 6-3, 7-6, 6-3, was dull and ordinary fare.

Tanner's sometimes electrifying first serve produced few sparks, but he got good depth on his second serves and volleyed solidly in stopping impossible dreamer DuPre, whose body finally paid the toll for the five long matches he played in his delightfully improbable trip to the semifinals.

Borg, who started the fortnight an 11-10 favorite to retain his title, has now won 27 consecutive singles matches at Wimbledon since Arthur Ashe beat him in the 1975 quarterfinals.

He is considered such a prohibitive favorite to beat Tanner Saturday and capture his fourth successive singles title - a feat that no man has achieved since New Zealander Tony Wilding in 1910-13 - that many London bookmakers have stopped taking bets on the outcome.

But Borg predictably downplayed suggestions that his victory over Connors, his longtime archrival whom he had routed in all three of their matches this year and has beaten in eight of their last 11 meetings, was the de facto final.

"Absolutely not. It is going to be more difficult on Saturday because I have now more pressure on me," said Borg, who historically has responded to pressure with a sure touch and eagerness that has produced his best performances.

"Everybody says that it is going to be an easy match, but I know it is going to be difficult. I just hope I can play as well as I did today."

When Borg dumped Connors in the final here a year ago, 6-2, 6-2, 6-3, Englishman Fred Perry - the last previous man to win the Wimbledon men's singles gles three successive years (1934-36) - said, "The way Borg was playing, if he had fallen out of a 45th story window, he would have gone straight up."

Today, he was even better. It took him three minutes less to win. And one got the impression that if he had fallen out of that same window, he would have fallen on Connors.

"I'm feeling much, much more confident, more comfortable now playing against Jimmy than three or four years ago," said Borg, who now has asserted clear superiority in their rivalry.

In their two previous matches this year, Borg won on clay at Boca Raton, Fla., 6-2, 6-3, and on cement, Connors' favorite surface, at Las Vegas, 6-3, 6-2.

"I'm hitting my ground strokes better now. Before, I was pushing the ball more, playing more soft balls, and I didn't put pressure on him. He was the one who put pressure on me, and I just tried to slice it back so that could make the mistake," Borg said.

"Now, I'm hitting through the ball, both sides. I'm not hitting so many short balls that he can come in on. I'm putting the pressure on him so he has to stay back at the baseline."

While most of the crowd, which included such diverse celebrities as jockey Steve Cauthen and Princess Caroline of Monaco, was stunned by Borg's superiority, Tanner was not.

"I've been practicing at the same club as Jimmy for the last couple of weeks, and I really felt he wasn't playing that well," said the 27-year-old native of Lookout Mountain, Tenn., who in two prior semifinals here was beaten by Connors (1975) and Borg (1976).

"He would have had to play very well to beat Bjorn, so I wasn't that surprised - particularly when he lost the first set. Bjorn has an awful lot of confidence against Jimmy now. He's pounded him the last few times they've played, and when he beat him in Las Vegas on cement, I think that was a key turning point in both their minds, because now Bjorn is content to stay back and play to Jimmy's backhand on a lot of points.

"When he does that, he's content to play the way Jimmy likes to play - slugging his backhand. He wasn't even hitting to Jimmy's forehand until he had to. So I felt that Bjorn showed he has so much confidence against Jimmy that he can challenge his strength, and play him almost any way he wants to . . ."

Connors came out looking as if he were ready to fight for his life, and, in fact, played well enough - at least in the first nine games - to beat almost any other opponent.

He started out charging the net, hurtling himself into shots, his feet and hair flying in the inimitably physical style that makes him the Raggedy Andy of tennis.

But it wasn't long before Borg knocked the stuffing out of the rag doll, discouraging his advances to the net, rocking him back on his heels with devastating service returns.

Borg hit out on everything - indeed, his only failing was a tendency to overhit some forehands - and pinned Connors to the baseline. That was where he wanted him, for no one outslugs Borg from the backcourt.

The first set was full of torrid shotmaking, but Borg grabbed the upper hand early. He held serve from 15-40 in the fourth game and broke Connors in the fifth, getting to his fourth break point with a screaming backhand down-the-line passing shot, then feeding Connors a short ball that he raked into the net with a backhand.

Connors froze, hunched his shoulders and extended his familiar middle finger toward the net. He stalked angrily to the changeover and pounded his chair with his racket before sitting down.

But he could never channel his anger into an assault on Borg, and gradually it turned to abject frustration. The Swede saved a break point in the next game and pulled out of trouble with two mighty serves, as he did often the rest of the match. Almost everytime he needed one, he reached back and exploded an ace (he served 11) or a service winner.

By the time Connors lost his serve again in the seventh game, he was getting punched out. He had invested incalculable energy in every point, having to win some of them two and three times, and still he found himself 2-5 down.

The second set was even more discouraging for Connors, because while he struggled in every game on his serve, Borg lost only one point in four service games. By now Borg was blistering Connors with his returns, scorching him with blazing passing shots down both lines, and coming in when he felt like it behind deep approach shots, to make winning volleys.

Connors did lead, 2-0, in the third set, Borg playing his only loose game to lose his serve in the first game. But from 0-2, Borg lost only 11 more points in the match, running the last six games.

In the end, Connors became almost as ragged as Borg was sharp. Precisely, 10 minutes after the last ball was struck, he piled into a waiting silver Mercedes with his mother, cornerman Lornie Kuhle, and a driver, and sped off - not even waiting to shower.

"Have you got a couple of minutes?" pleaded one reporter, encouraging Connors, who has been incommunicado since he arrived in London, to end his boycott of the press.

"A couple of mimutes are passing right now," snarled Connors, slamming the door as the car pulled away. He was expected to be back in L.A. where his wife is awaiting the birth of their first child, before the day was out.

Compared with the scintillating shotmaking of the Borg-Connors match, Tanner's 1-hour 43-minute victory over DuPre was bald and uninspiring. It was hit-or-miss tennis, few points going beyond five shots. Many were decided either by return errors or first volleys. There was hardly any stroke play.

There were only two service breaks in the match, Tanner breaking DuPre with a buzzing backhand cross-court pass in the eighth game of the first set, DuPre bungling a couple of volleys to lose his serve at love in the eighth game of the third set.

Tanner won the second set in a tie breaker, 7 points to 3, forcing a backhand volley error with a hard forehand on the first point and making an nice forehand down-the-line pass for 6-3.

Those were the only points that went against serve, as Tanner got in four of five first serves - a much higher percentage than he enjoyed for the whole match. CAPTION: Picture 1, Jimmy Connors' Centre Court efforts were fruitless. UPI; Picture 2, Bjorn Borg clubs a topspin forehand in semifinal. AP