"I still don't think of Bjorn Borg as a grass court player. The fact is, nobody's entirely sure why he's done so well at Wimbledon for so long. I'm completely certain I can beat him here." -- John McEnroe, world's second-best tennis player

"A lot of players can beat Borg if they serve and volley well and play on a fast surface. Connors can beat him. McEnroe can beat him. On clay, you almost can't beat him. But here at Wimbledon, why not?" -- Rolf Gehring, winner over Borg two months ago

How do you beat Bjorn Borg at Wimbledon? That question is tougher than it sounds. And it's the query that will be posed more often than any other in this final week of the championships.

The quarterfinals for women begin Monday with the men's quarters to follow Tuesday. Although 16 players are still competing here, one easily outshines the others -- Borg. The 25-year-old Swede is all the more appealing because he remains a tennis mystery.

Perhaps the most puzzling brain teaser in tennis is why Borg has had his greatest triumphs, has built the cornerstone of his reputation as perhaps the greatest player in history, in the one place, and on the one surface -- grass -- that should suit him least.

Borg's incredible streak of five consecutive championships (the most since white-flannel days) and 39 straight matches (the most ever, surpassing Rod Laver's 31) is all the more compelling because, at least in tennis theory, he should be so vulnerable.

In tennis, surface is everything. The clay of the French Open, the grass of Wimbledon and the inbetween-speed composition surface of the U.S. Open may not constitute three different sports, but it's close.

Perhaps no analogy is apt, but for a tennis player to be the best on earth on both clay and grass might be comparable to a track man being able to win both the 100-meter dash and the 1,500 meters in the Olympics. In both cases, the body type, the particular skills and the training techniques that apply to one should almost rule out excellence in the other.

That's why Borg is such an anomaly. And that's why, year after year, his peers here are amazed that he can build the greatest streak in tennis history -- and perhaps in the history of all individual sports -- under conditions that seem antithetical both to this style of play and his basic temperament.

Every tennis buff knows that Borg learned the game on the slow, high-bounce surface of clay where defense, patience, stamina and cautious strategy are gospel. Every aspect of Borg's game was born of clay and geared to it. And, throughout tennis history, being bred on clay has meant, at the highest reaches of the game, having feet of clay.

Clay grabs the ball, slows it and makes it sit up so it can be hit easily. To gamble for an outright winner is folly since a nimble opponent, roaming the baseline, will usually run it down.

Grass, of course, is fast, hard, slippery and tricky stuff. It rewards every gambling shot in the game: the violent serve, the hooking spin serve that yanks an opponet into the cheap seats and leaves the court naked for an easy volley at the net. Grass is what tennis exists for, since it emphasizes all-around athleticism. If you have power, grass rewards you. If you have the touch for drop shots, angled volleys and tantalizing half-lobs, grass helps you, too. And if you have speed and maneuverability, you are also rewarded because grass is such a treacherous footing that it immediately exposes the klutz.

Borg's two greatest clay assets, the bedrock of his tennis, are his utterly dependable topspin ground strokes, especially the bludgeoning two-handed backhand, and his ingrained preference for counterattack, especially the precision passing shot when a foe is greedy enough to come toward the net.

Yet the phase of tennis that should mean least on grass is the ground stroke. As John McEnroe says, "Against Borg, I'm going to come in most of the time. There aren't going to be a lot of long rallies if I have my way. I can't outlast Borg from the back court, but on grass I don't need to."

And, the tenor of mind that least suits grass is the instinct for counterattack. The very nature of the grass bounce favors the attack. Hit a hard, low skidding shot and the poor fellow back at the baseline ought to have the devil's own time managing the perfect passing shot. As Arthur Ashe has said, 'Nobody in 30 years has been able to win on grass from the baseline hitting topspin -- until Borg."

Given all this, why has Borg won 39 straight matches, and what does that tell us about how he can be beaten and when it might finally happen? First, Borg, the ultimate practice animal, has, since 1977, developed one of the best serves in the game. And each year his volleying, especially with the backhand, has improved.

Borg has learned to blend attack and caution. His first serve is a free chance to attack and win; if he pounds it home, he follows up his advantage and finishes the point quickly. However, unlike other serve-and-volley players, Borg is not unglued by having to depend on a less menacing second serve, since he is perfectly willing then to rely on his baseline game.

Second, Borg's topspin passing strokes are uniquely effective because they dive as they cross the net.

His foes find that they must guide their volleys upward, rather than punch them downward for authoritative put-aways. Thus, Borg is able to use his great speed and anticipation to chase down volleys, then tee off on the passing strokes that, at Wimbledon, have become his slashing trademark. Borg has other, less visible, advantages. One is a strange temperamental preference. "The only time that I really want to take chances and attack is when I am behind," says Borg. "That is just the way I am. I say, 'I am going to lose,' and I start hitting out more and going for lines." s

This native daring unnerves opponents. Since they think of Borg as essentially defensive and patient, it stuns them to see him attack wildly and successfully. They sense that they have unwittingly pricked the lion's paw.

Also, in tie-breaker games (of which he has had eight in the last four years) and other key games, Borg has a knack of blending strategies which seem contradictory. "I am best on big points," he says. "I relax most then. sThat's when you have to go for the ace. And, you know, for some reason on the important point, I always seem to hit my first serve in."

Here Borg gives a tiny metaphysical grin and twinkles those close-set eyes that must make him seem like a cyclops to opponents. Having created that impression of daring, Borg then reverses form by playing the rest of the game hypercautiously. Just as he gambles when he's losing, Borg follows a gut impulse to play strictly by the house percentages when the game, set or match is teetering in the balance.

Obviously, Borg has struck on subtle ways to make the pressures of his own winning streak, the flashbulb glare around it, work against his victims.

When they are winning and smell Borg's blood, he unnerves them by contradicting their expectations and unleashing his best attacks. The more vital his first serve, the more Borg demands of it: another tactic that would convince an opponent that he was facing the ice-in-the-stomach daredevil.

Finally, Borg realizes that the tension of being the man who breaks Borg's streak is -- at the crisis moments -- more crushing on the challenger than it is on the champion. Entering a match, Borg's foes are loose and aggressive: "They have nothing to lose. I remember when I was young and played like that," says Borg. But, in a fifth set, in a tie breaker, the dynamic changes. The man with a realistic here-and-now chance of snapping the Borg streak knows he has a once-in-a-lifetime opportunity, a moment that may surpass all the rest of his career combined.

Despite all this, Borg can be beaten here this week. That's "can," not will. You don't step on Superman's cape and you don't bet against Borg at Wimbledon. Nonetheless, Borg's streak becomes more precarious all the time.

For one thing, Borg's thought processes, once so hidden, have become better known with the years; the book on him has gotten better. Borg may be unconventional, but he is not unpredictable.

Also, everybody now knows Borg's only weakness -- such as it is: he can be pulled off the court by a lefthander who has a good spin serve to his backhand side. Basically, if you aren't left-handed and don't have an excellent serve, you can forget about beating Borg. It's the first prerequisite.

For instance, the right-handed Borg has been knocked out of the U.S. Open nine times: once by injury and six times by southpaws. Also, in the last three years, since the streak has been an issue, Borg has been scared to death in five-set matches four times, and three were by southpaws: Victor Amaya (1978), Roscoe Tanner (1979 final) and McEnroe (1980 final). vRighty Vijay Amritraj in 1979 was the exception. In the 1977 final, Connors, a lefty, also had a five-set war with borg.

Should Connors beat Amritraj in the quarters and Borg beat Peter McNamara (a right-hander he has always handled easily), Connors would probably have to adopt a McEnroe tactic when he meets Borg in the semifinals: the soft drop shot.

It is necessary, both tactically and psychologically, to yank Borg away from the security of the baseline at times of your choosing, rather than his. Also, Connors' passing shots are a strength that he never uses since he has never sucked Borg to the net. It may, however, be beyond Connors' combative temperament to use a finesse strategy. Should McEnroe meet Borg in the final in a glorious resumption of their two classics of 1980 -- the 55-game Wimbledon final and the 55-game U.S. Open final -- it is probable, according to sources close to the McEnroe camp, that he will use more of the one tactic he hasn't tried much: the dink and lob.

Lastly, this year's Wimbledon has an extra dimension. Both McEnroe and Connors, who knows whether perhaps consciously, have stirred up the normally formal and decorous atmosphere of the All England club by creating controversy. McEnroe has been the incredible sulk for a week, blasting everything about Wimbledon: the place is Borg's shrine and he's been going around spitting on the stained glass windows. Perhaps McEnroe knows he will only be loved here in the role of gallant loser to the superhero Borg, so, he has chosen the only alternative -- the role of heathen defiler. Connors has also joined in the stirring of the waters, claiming that Wimbledon officials are biased in favor of Borg because in his last 30 matches, the 25-year-old Swede has never been forced to play on the scruffy, upset-prone outside courts.

In all Borg's championships, he has never had to face two players as good as Connors and McEnroe in the same year. He's played one great challenger per year, but never two bona fide big boys. This year, he probably will.

All that may not add up to the end of a streak that, before it is over, may be regarded as one of the unmatchable accomplishments in all of sports.

"If you ask my advice on how to beat Borg," said Rolf Gehring, one of three players to defeat Borg in 1981, "I would say, 'Serve a lot of aces, and have plenty of net cords and line balls.'" This, of course, is the worst strategy of all. At Wimbledon, a monopoly on luck is held by Borg. He makes sure it's always that way.