How do you beat Bjorn Borg on a clay court? That is a deep metaphysical question the 23-year-old Swede's rivals on the professional tennis tour have been pondering for some years now, and the answer seems to be as elusive as the riddle of the Sphinx.

For Vitas Gerulaitis, it is a question of considerable urgency. "Broadway Vitas" plays defending champion Borg on Friday in the semifinals of the French Open, the tennis world's most prestigious clay-court test.

The victor will oppose the winner of the other semifinal, between Jimmy Connors and Victor Pecci, for the $49,000 top prize Sunday.

Many fine tennis players has never beaten world champion Borg on any surface. Gerulaitis, ranked No. 4 in the world, is one of them. He is 0 for 11 against his frequent practice partner and the frustration of never having beaten him is mounting like that of a cat who chases furiously but cannot quite catch his tail.

Gerulaitis has come close to vanquishing Borg several times, most notably in their classic 1967 semifinal on Wimbledon grass, when Borg won after four hours of thrilling thrust and parry, 6-4, 3-6, 6-3, 3-6, 8-6.

But close is not good enough, and so Gerulaitis and his coach, Australian Fred Stolle, have been plotting strategy.

"I feel that when Vitas goes out there against Borg, he plays exhibitions with him," said Stolle. "He always plays the same. When a guy has got you 11 and zip, you shouldn't be nervous about it, your should be figuring out some different way to play him."

Everyone agrees that Gerulaitis has a better chance against Borg on a surface faster than the red-clay center court of Stade Roland Garros here, but that is the stage of the moment. Gerulaitis will do his best, even though it is not suited to his serve-and-volley-preferences.

He has played Borg on clay only twice. The most recent was in the final of a WCT tournament in Monte Carlo in April. Borg murdered him, Gerulaitis showing the aftereffects of a week of late nights and early mornings out on the town.

On that occasion, Gerulaitis tried to rally with Borg from the back court, an approach that Stolle has sensibly deemed unworkable.Nobody beats Borg from the baseline, as is evidenced by his 9-0 record against Guillermo Vilas since 1975, his lifetime records of 13-0 against Eddie Dibbs, 12-0 against Harold Solomon and 10-0 against Corrado Barazzutti, some of the most accomplished back-court players of the modern generation.

This time, Gerulaitis will try to chip his backhand and go to the net as often as possible, as he did in beating Vilas in an epic Italian Open final a week and a half ago.

He will try to draw Borg into the net with dropshots, especially to the slightly limited reach of his two-fisted backhand, hoping to move him into awkward positions where he will be vulnerable to an offensive job or passing shot.

And he will try to constantly change pace, taking a leaf from the notebook of the Italian, Adriano Panatta, the only player who has ever beaten Borg at Roland Garros, in the fourth round in 1973 and again in the quarterfinals in 1976.

"Panatta hits a big serve, cracks a ball or two, then drop-shots or gives him a soft one. He never plays four balls the same way and never gets in a rally with they guy, which is the way you have to play Borg," said Gerulaitis.

"It's difficult to play a different ball every time, to constantly change the rhythm, but i'm convinced that's the only way to beat the guy. Connors, even though he hits harder and harder as he plays Borg, still hits basically the same shot, and eventually Bjorn gets onto it. He runs everything down and bashes it back just as hard. I think it's impossible to outhit him."

Borg has won Wimbledon three years in a row on grass, a surface experts once thought he would never be able to adjust his game to. He has won indoors and on hard courts. If they played tennis tournaments on hot coals or tapioca, he would win them too.

But he is still at his lofty best on the slow clay on which he was reared. Here he can outsteady the most persistent baseliners, and break the heart of net-rushers with his incomparable passing shots off both wings.

The ball bounces high on clay, which gives Borg a good crack at it on his ground strokes, and time to run down shots that appear to be sure winners to the untrained eye.

Because he hits most of his shots with high-bouncing top spin, his opponents are constantly stroking the ball around their shoulders or eyes, a height from which it is difficult to generate power. And because he is quick and strong enough to beat anybody in side-to-side rallies, Borg can play shots high over the net and relatively short, giving himself much less of a margin for error than, say, Connors, who pounds his ground strokes more riskily flat and low over the net.