Now that concerns about Bjorn Borg's recently inflamed right knee have faded away, the consensus of experts is that the unflappable Swede is about to conquer the one notable bugaboo of his career and win the U.S. Open tennis championship on Sunday.

Borg, 24, has won Five French Open titles on Parisian clay and five successive Wimbledon titles on London grass. The Open was played on artificial clay at its former home in Forest Hills in 1975-77, and is now contested on a rubberized asphalt hard surface trade-named Decoturf II at the National Tennis Center in Flushing Meadows.

Failure to win the Open is the only blot on Borg's astonishing record, the only conceivable argument against canonizing him already as one of the game's all-time greats. He knows this, as surely as Napoleon knew things would have been different if he won Waterloo.

Just as Wimbledon -- which is cool, quiet and dignified like Borg -- has been his grandest showcase, the noisy, sticky, hot and frenetic Open has been his most troublesome stage.

He reached the final in 1976 and 1978, losing both times to Jimmy Connors, who unbashedly loves New York and thrives on the Opens raucous, prizefight atmosphere.

In 1977, Borg defaulted with a shoulder injury he had incurred while water skiing. In the '78 final -- the last time Connors beat him in a match of significance -- he had a badly blistered racket hand. Last year, when Roscoe Tanner bumped him in the semifinals, Borg went into the match in a bad frame of mind, carrying an uncharacteristic mental block about returning Tanner's left-handed laser beam service under floodlights.

"But most of all, I just haven't played well in this tournament -- never my best tennis for two weeks," says Borg. "I don't know why. Some tournaments you are playing well and others you aren't. It's something you can't explain."

It's not the airplanes roaring overhead, practically rattling the fillings in ones teeth, not the bustle and confusion and loud distractions of the Open, Borg says.His concentration is as intense and total as any players. He can block out these maddening intrusions.

Nor is it the surface, which is indigenous to America and took him several years to master. He became comfortable on it, winning a number of tournaments around the U.S. on hard courts.

"It takes a little while to get used to this surface, but I know I can play very, very well on it. That is no problem for me," he said. "Three years ago, maybe, it was difficult because the Americans grew up on cement courts but in Europe this did not exist. I had to practice and play matches on this court to see what it is all about. But I know I have the game, the shots, to play well on it."

Does he feel confident, then, that he will end his peculiar famine in America's premier tournament?

"I hope so, because that is my biggest goal right now, to win the U.S. Open," said Borg. "If I play well, I know I can do it. But maybe I never will. I mean, you never know. I just know that every year, as long as I play the circuit, I'm going to try, and I just hope one year I'm playing 100 percent."

Coming into this year's combination shootout, Borg had lost only two matches in 1980: to Guillermo Vilas in the Nations Cup in Dusseldorf, and to Ivan Lendi in the Canadian Open three weeks ago. It was there that Borg defaulted with a tender knee after winning the first set.

His ailment, an inflamed bursa sac that is commonly called "runner's knee," was incurred during his honeymoon in July, while jogging on pavement in tennis shoes rather than cushioned running shoes. It caused Borg considerable pretournament anxiety. He took five days off, resuming practice only five days before the tournament.

But since then, he says, he has had nary a twinge. The way he sprinted and changed directions in beating Yannick Noah on Monday dispelled any lingering doubts about his fitness. His footwork was swift and splendid choreography, a sporting ballet. Bad knee? Bah, humbug.

Borg also has gotten over the psychological hurdle of playing a night match.

He beat Australian Peter McNamara under the lights last weekend, and in the last two sets was better than "Saturday Night Live."

Next in line is a quarterfinal against Tanner, who last year thwarted Borg's aspirations toward a French-Winbledon-U.S.-Australian Grand Slam at the same stage. This time, the outcome likely will be different.

Tanner has not had the year he did in 1979, when he extended Borg in a thrilling, five-set Wimbledon final, and rose to No. 5 in the computerized world rankings. Although he has worked hard in recent weeks to get back in shape, Tanner has had a largely indifferent year and has slipped to No. 15

This time they will play in daylight. Borg is less worried about returning Tanner's fast, deceptive blurs when he can see them. Tennis balls travel faster through hot air than cool, but the afternoon humidity of Flushing Meadow will slow them a trifle.

Moreover, the court plays slower in the heat of the day than at night, and Borg is much less likely to wilt in the oppressive mugginess than Tanner.

With his low pulse rate and blood pressure, Borg seems impervious to the heat. He looks to be the strongest, fastest, freshest man in the tournament.

The stadium court has been resurfaced since last year, and has a slower higher bounce. This favors Borg with his peerless high-bouncing, topspin ground strokes.

"They seem to have made the court for Borg rather than for the Americans," said grumbling defending champion John McEnroe, who played his heart out and lost by the breadth of one of Borg's stubby whiskers -- which are missing here -- in a majestic Wimbledon final. "The conditions are perfect for him."

Most of his colleagues believe it is only a matter of time before Borg wins the Open. They reckon that his annual frustration here has been a quirk of history, a curious run of bad luck: the flip side of the good fortune he has enjoyed in winning a record 35 consecutive matches and five straight titles at Wimbledon.

"You need a few breaks to win any of the big tournaments.In a few of his close matches at Wimbledon, you got the feeling God was looking down and saying, 'He's a good guy; let him win this one,'" said Vitas Gerulaitis, last year's Open runner-up, who has a 0-19 career record against Borg and was upset here by Californian Hank Pfister. "In the Open, he just hasn't gotten those breaks."

Said McEnroe, who would relish a return bout with Borg: "I'd be a lot more surprised if Borg never wins the Open than if he wins it this year."

Tanner reasons that Borg is virtually unbeatable on clay (excluding a couple of injury defaults, Borg's loss to Vilas at Dusseldorf was his first on clay since 1976), but still more vulnerable on hard courts than on grass.

"He does some things where a hard court hurts him. It's more difficult for him than other surfaces," said Tanner. "If he comes in to volley, he can't hit that short one that he does on grass. It sits up and gives the opponent a whack at it, whereas on grass that shot dies for a winner."

Still, Borg's unorthodox game is effective on any surface. The exaggerated topspin off both sides, which makes the ball dip and leap high off the court, has several advantages:

1) He can hit with a higher arc over the net, with more margin for error, than players who hit flat or underspin shots which must go lower over the net if they are to land deep in the opponent's court.

2) He can hit the ball exceptionally hard, relying on spin rather than gravity to deep it in court.

3) He has to worry less than other players about hitting short balls, because even when his shots fall barely beyond the service line, they are difficult to attack. The high bounce means that opponents are constantly hitting up around their shoulders, and unless they can take a ball on the rise and slice it -- a tricky feat -- it is difficult to hit a good approach shot off what one colleague admiringly calls "Borg's high-bouncing snakes."

Some stars of an earlier era, including Australians Frank Sedgeman and Lew Hoad, say they would have darted in and plucked Borg's loopers out of the air on the volley, but that is easier said then done. Topspin is deceptive and difficult to volley, and Borg can hit with such savage pace that he gives even normally assured volleyers fits.

In short, Borg plays with a much greater margin of safety than players such as Connors. His ground strokes are so steady that he can move the ball around, gradually hitting harder and harder, until he forces a short ball to punish or until he jerks the opponent hopelessly out of position.

When Borg takes control of the tempo of a point, he almost invariably wins it. He is one of the best instinctive geometric minds since Euclid, and makes sure that the angles are always in his favor.

If he is not playing well, Borg can fall back into a defensive style and keep the ball in pay until the opponent makes an error. But essentially, he has developed a backcourt game that is so sophisticated he can atttack with it at will.

"Basically, he positions himself two or three yards behind the base line and just lets his topspin take its natural course," says Arthur Ashe, the consummate thinking man's tennis player. "He doesn't even have to hit the ball that hard all the time, because it comes over and dips.No matter what surface you play him on, if you go to the net, you're always volleying up.

"You really can't put the ball away that way, and he's so quick he just runs everything down and is in postion to hit a passing shot. And his ground strokes are so damn good, there's no way to beat him from the backcourt. If you let him play the match between the service line and the back fence, he'll kill you. He's the absolute master there. You have to bring him in somehow."

Many other players hit with topspin these days -- one needs only look at all his European clones to see Borg's impact on the upcoming generation of tennis players -- but nobody does it better. He hits harder and with more consistency than any other player, and also has developed an outstanding serve and serviceable volley.

It used to be against his nature to play a serve-volley game, but he did so throughout Wimbledon this year, and has gone to the net much more in this Open than in previous ones. Borg is now an all-court player, not just a base-line attacker and counterattacker.

A good serve-volley player can still trouble him, especially if he has a good serve wide to this two-fisted backhand. McEnroe and Tanner are praticularly difficult opponents for Borg, because their lefty slice serve tails away from his backhand. But eventually, Borg can get on that serve and start cracking return winners, as he did after McEnroe served him off the court for the better part of two sets at Wimbledon.

"It's not that his passing shots are unpredictable," said Dennis Ralston, the former U.S. No. 1 player who coaches Tanner, "We generally have an idea what he's going to do. He has certain tendencies and patterns, just like other players. He doesn't fool us that much.

"But the topspin is such that it's dipping and diving, and he hits it hard, so that's a tough, tough ball to volley. It's not a question of disguise, but a lot of times he just hits it too good."

Several players have a number of strategies they try against Borg. Ashe, for example, suggests roaming far out on the base line to serve, instead of staying near the center mark, keeping the serve wide and angled to the backhand, drop shotting to the backhand and covering the down-the-line reply, approaching down the middle instead of on the wings to cut down Borg's angle, and hitting short, soft "teasers" to bring him to net. But these tactics are much easier to plot than to execute effectively.

Borg, for this part, says he has no idea how the split-second reactions that determine where he will hit a given ball germinate. They come naturally, as second nature.

"You don't really know yourself where you're going to hit the ball until the moment you make contact," he said. "You don't think to yourself, 'If he approaches down the line. I'll hit cross court." Maybe some players do, but as the point develops, you know what to do. You don't think, you just do it.

"Maybe it comes from doing it a million times before."