By Bjorn Borg earned $424,420 in official tournament prize money and competitive bonuses last year. He also played 15 one-night exhibition matches for between $10,000 and $25,000 apiece.

Then he got around to his offcourt business interests and started making some serious money.

"You could say that Borg is tennis's answer to the Six Million Dollar Man," said Bud Stanner of Mark McCormack's Cleveland-based International Management Group, which merchandises Borg and other sports celebrities.

"It's safe to say that Bjorn is making considerably more than $1 million a year, more of it off the court than on," said Stanner, who is, of course, competing with other agents and managers for the right to peddle the Borgs of the athletic world.

Reasonable approximation of Borg's annual income ranges from $1.25 million to $2 million.Jimmy Connors is said to be guaranteed at least $1.25 million in oncourt revenues this year, plus substantial amounts from Robert Bruce clothing, Wilson rackets, Canada Dry beverages, and other endorsements and appearances.

Twenty-two tennis players (17 men and five women) had competitive winnings of more than $100,000 last year.

All of them made additional offcourt income, aside from investments, but for such heavyweights as Wimbledon champions Borg, Connors, Arthur Ashe, Billie Jean King, John Newcombe, Chris Evert, Rod Laver and Stan Smith, plus some Europeans and South Americans who command huge commercial fees in their homelands, the real financial bonanza is in commercials and product endorsements.

"All the tennis players are walking billboards. They are extensions of the outdoor advertising industry," wrote John McPhee in a 1972 book about Wimbledon. "Almost everything they drink, wear and carry is an ad for some company . . . Players endorse nets, gut, artificial court surfaces and every item of clothing from the jock on out."

Not to mention mattresses, sheets, towels, patent medicines, soft drinks, cars, watches, cameras, resorts, board games, books, and a catalog of other goods and services that suppliers wish to present to the affluent, trendy and ever-growing tennis audience.

"Demographies" is the magic word. Poll-takers tell us that more than 80 million people in America alone play tennis "from time to time," 20 million of these at least a couple of times a month. A similar boom is on in Europe, Japan and other parts of the world.

The tennis market has the appetite and resources for conspicuous consumption. Endorsement opportunities have increased almost geometrically the last five years, and the fees paid to players for personal services and appearances have spiraled along with tournament prize money.

Newcombe, who is by now a one-man miniconglomerate, and fellow Australian Layer have demonstrated that players with the proper reputation, personality and advisers can capitalize and expand their income even after they have passed their prime on the court.

But Borg, the reigning Wimbledon champion who will not turn 21 until next month, is perhaps the outstanding example of head-to-toe merchandising.

He is paid $50,000 a year for instance, to wear a headband advertising Tuborg; the Danish brewery, every time he walks on court in public.

Since athletics are not permitted to pitch alcoholic beverages in many places, the terrycloth headband that keeps Borg's shaggy blond locks out of his eyes bears the name "TUBORG" in big letters, with the words "Orange Soda" in smaller print below.

Borg also is paid more than $100,000 to play with Bancroft rackets in the United States (Donnay rackets in Europe).

He is even paid to endorse the gut with which the rackets are strung: a brand called VS. (The fee for this is strictly peanuts, about $2,000, but Borg gets as much gut as he can use.

Borg also realizes approximately $200,000 in tennis-clothing contracts - Fila shirts, shorts, socks and warmup suits in the U.S., Jockey tennis wear and Wertex hosiery in Europe.

He gets in the neighborhood of $50,000 for wearing Tretorn tennis shoes everywhere (he used to wear Tretorn in the U.S., Adidas in Europe), and something in excess of $25,000 for having a Scandinavian Airlines System patch sewn on the shoulder of his tennis shirts. (Other players such as Wojtek Fibak wear their national airlines insignia on their sleeves, while Adriano Panatta and Ilie Nastase use that space to advertise Marlboro cigarettes.)

Borg also receives more than $10,000 for doing radio and print commercials for Nutriment food supplement; more than $50,000 for peddling Saab automobiles, and another $50,000-plus for his association with a soon-to-be-built resort in Mexico called "Camelot."

In addition, he endorses Dextrasol glucose tablets and Kellogg's cereals (presumably not to be ingested simultaneously) in Scandinavia, Lois jeans in Spain, and a tennis board game throughout Europe.

In Scandinavia, he has his own line of comic books and bed linen. A Swedish firm called Chirls Intersales markets busts of him in Sweden, where he is considered, as Stanner says, "kind of a national resource."

In the United States, Factors, Inc., is preparing to market an iron-on transfer of Borg's likeness, suitable for tee-shirts. Franco Manufacturing is coming out with a Borg line of towels and household linen.

According to Stanner, a player in the class of Borg, Connors, Ashe, Evert or King can expect to realize the following annual revenues from various categories of endorsements:

$100,000 for rackets.

$200,000 for clothing.

$25,000 to $50,000 for shoes.

$25,000 to $50,000 for on-person identification such as a shoulder patch or headband.

$10,000 to $25,000 for radio and print endorsement of a soft drink or food product.

$25,000 to $75,000 for an automobile, depending on "tradeouts." (The manufacturer generally makes a car available to the endorsing wherever he goes, and sometimes to family members as well.)

$50,000 to $100,000 for club or resort affiliation, again depending on tradeouts. Players often are compensated in rial estate or condominiums, but Stanner says, "It's always better to get cash . . . we try, but sometimes it's not possible."

$25,000 to $75,000, including residuals, for a television commercial that is aired nationally in the United States by a mass media-oriented company (Borg for Bancroft; Evert for Clairol; Ashe for Beautyrest, etc.).

A guarantee of $5,000 and up, against royalties, for towels, tee-shirts, board games and other random products.

"You figure it must be worth at least five or 10 grand to make it worthwhile for a player like Borg," Stanner says. "Of course, players who are less in demand and less recognizable may want to do things for less."

IMG and the other agents and managers who represent tennis players do not pretend to be charitable institutions themselves. IMG, for instance, takes 25 to 35 per cent of a client's total annual income as its fee.

In addition to lining up the extras for clients, the firm also offers tax, investment and personal management services.

Tax advice is a major factor. Borg, for example, now lives in Monte Carlo because income taxes in Monaco are, as Stanner puts it, "minimal." Borg's neighbor in the same apartment building is the Argentinian player, Guillermo Vilas. Other players have changed their country of residence for tax purposes.

"I would not hope to make half to three-quarters of my oncourt income in endorsements and appearances," said Harold Solomon of Silver Spring, Md.

"I guess I earned about $300,000 on the court last year, so I expect to make $150,000 or $200,000 over and above that. That's what I'd hope for."

Do the players genuinely use and endorse the products to which they lend their names? Most do, some don't.

Laver, for example, signed contracts a few years ago to play with Chemold metal rackets in the United States and Donnay wooden ones overseas. But his game became erratic, so he went back to the old, trusty Dunlop wood rackets he had used since boyhood, disguising them as best he could as Chemolds and Donnays with paint jobs and racket covers.

Charlie Pasarell, asked one time if he liked Adidas shoes, gave the classic reply: "I do, in Europe." In the U.S. he liked Uniroyal Pro-Keds.