Bjorn Borg and Ilie Nastase, the stolid Swede and the moody-bluesy Romanian, are two shining lights of European tennis who could scarcely be more different.

Borg, 22, is thoroughly Scandinavian, icy and impassive. His pulse rate has been measured at 35 beats per minute. His deep-set eyes seldom reveal emotion on the court. When he is at work, playing, all he cares about is taking care of business.

Several weeks short of his 22nd birthday, Borg already has accomplished far more than any previous player his age in the history of the game: two Wimbledon titles, two French Opens, one Italian, three U.S. Pro Championships. In 1975, at age of 19, he led Sweden to the Davis Cup.

Last year, he won 12 of 20 tournaments, 78 or 85 tournament matches. Most knowlegeable observers ranked him No. 1 on the world.

Even though he defaulted Friday from the semifinals of the $200,000 World Championship Tennis (WCT) Finals here - a tournament he won the last time he played, in 1976, after being runner-up in 1974-75 - he is ofyear.

He has beaten Connors in two of three meetings, routing him in the last, at Tokyo last month. He won two of the five WCT tournaments he played, plus the $200,000 Tournament of Champions. His career record against the other seven players who qualified for the WCT playoff, all among the cream of the world class, is 41-12.

Before defaulting, Borg ravaged Dick Stockton, 4-6, 6-2, 6-1, 6-0.

Nastase, 31, is thoroughly Latin, mercurial and high-strung. His pulse rate has not been measured, but he has the metabolism of a hummingbird. His dark eyes smolder with passion, his delicate features are a playground for expression.

On the court, Nastase is tennis man-child in a promised land, alternately playful and possessed, graceful and neurotic. He is tormented by every slap of fate, real or imagined. His eccentricities surface in the heat of battle, opponents and officials become diabolical enemies in the tumultuous scenes only he can create.

"Nasty" has not won a Grand Slam singles title since the French Open of 1973, the lone year he was ranked No. 1 in the world, nor a significant international title since Grand Prix Masters of 1975 (fourth in five years).

He was a meek runner-up to Borg at Wimbledon in 1976. His explanation: "I was too concentrating. I say nothing, do nothing for two weeks. I behave perfect. This is not Natase; if I cannot show temper, I cannot play."

He already has won $111,841 in prize money this year, but was lethargic, heartless, painfully uninspired and uninspiring against Eddie Dibbs in a first-round match Thursday night, losing 6-3, 6-3, 6-0 in 84 minutes.

"Borg and Nastase are both very talented players physically, but they're at opposite ends of the spectrum as far as the mental side of the game," said Vitas Gerulaitis, an unabashed admirer, who will be playing for the $100,000 top prize afternoon.

"Nastase is the most gifted player of all, without question. He can do things with the ball that I've never seen anybody else do, or even think of doing.He makes shots nobody else can even imagine, but as far as concentration capacity, I don't know. I don't think he can do it anymore.

Borg? He's just all concentration, all work. Rarely does he have a lapse. Nothing upsets him. You'd have to say he and Nastase are pretty much opposites."

The contrast was graphic during a practice session one afternoon last week, Borg and Nastase are working out together at Moody Coliseum. Nastase has six rackets with him, strung in an unconventional way, with the vertical strings laid over the crosses instead of alternately under and over them. This is an attempt to simulate the exaggerated spin of the tournament-banned "double-strung" or "spaghetti" racket, but with only one set of strings.

Borg and Nastase are playing points that, if the Coliseum were full, would bring the house down. The few people watching are moved to applause frequently. Nastase grins and plays to the meager gallery. Borg, as usual, is undemonstrative.

On one point, they run each other all over the synthetic court, sprinting flat-out. Borg pastes a forehand down-the-line, a sure winner. Nastase, off like a zephyr, runs it down and flicks a backhand cross-court at an impossible angle.

But Borg is also capable of astounding sleight-of-foot. He changes direction, gets to the ball, and coaxes a trumping Nastase's masterpiece. Nastase glares at the rafters and swears loudly. Borg's expression does not change.

Such scenes are repeated again and again - splendid points, Borg winning perhaps three-fifths of them, Nastase does all the animated cartooning, the laughing, the cursing, the chattering to bystanders. Finally he cries, "I'm tired," and dramatically lies down in the backcourt.

Borg waits impatiently for him to get up and serve. Nastase finally does. He takes a deep breath, runs a hand through his lank, matted black hair. "Ready, darling?" he asks in falsetto, and this time even Borg cannot help but grin.

"You getting angry, Dibbs?" Nastase peers at Borg, fully aware that Eddie Dibbs is looking on. "I drive you crazy, Dibbs my dear." He wallops a forehand for a winner and puffs out his chest, imitating a gorilla, and bows again to the assembled onlookers.

Borg also has a formidable stack of rackets at courtside, but he uses only one for the entire practice. He has them strung at 80 pounds tension, tight as boards Jack Kramer says that if a normal player used Borg's rackets, his arm would quickly fall off.

If Borg plays with a jet-age racket, some people consider his unorthodox sytle to be futuristic tennis.

"He hits every shot hard, about six feet over the net, and it lands six feet inside of the baseline because he puts so much tops spin on it," Australian player Barry Phillips Moore once observed. "Although he doesn't really think about it, Borg's game is a case of advanced technique."

Borg is not a deep thinker or articulate analyst, but he has an instinctive grasp of the complexities of tennis, a mind that absorbs and stores information readily. He says he can play one set against a new opponent and know exactly what he likes to do. He might not play him again for two years, but can then produce his technical and tactical profile with computer-quick recall.

Borg is also diligent worker. If Nastase is a temperamental artist, painting the most ingenious brush-strokes with his racket, Borg is a more earnest and exceptionally skilled artisan, fashioning records that wil last longer.

"I would like to think that Nastase can still win a big tournament because he's a friend of mine and it's great to have him in the game. He's done a lot for tennis. Good or bad, he's created a lot of interest," says Gerulaitis.

"But I just don't think he can go a whole week, or two weeks, to win Wimbledon or the U.S. Open. Eventually he's going to meet a Connors or a Vilas or a Borg, who keep knuckling down . . . "