Bjorn Borg and Jimmy Connors, the Hertz and Avis of the U.S. Open tennis championships, were practicing on adjacent courts at the new National Tennis Center in Flushing Meadow Park Monday, the day before the start of America's premier tournament. Perhaps 100 onlookers were watching them, none more intently than their respective coaches: Lennart Bergelin and Pancho Segura.

"Jeemy's got to win. I don't see how he can lose. He's too solid on hard courts," said Segura, the bronze-skinned Ecuadorian who is back in Connors' corner at least for a few days - after an absence of two years. He said it loudly enough for Connors to overhear as he sparred with Eddie Dibbs, the most improved pro of this summer.

Few others consider Connors more likely to succeed here than Borg, who is seeking the third leg of the French-Wimbledon-U.S.-Australian Grand Slam. But Segura is a positive thinker, as Connors demands of those in his camp. Pancho is the sort of handler who calls his man "Champ" even after he has lost the title.

"Throw the ball a little higher. Lean into it." he yelled to Connors, who has been working furiously to add oomph to his serve in a desperate effort to close the gap that has arisen between him and arch-rival Borg, winner of five of their last six meetings, including the last two Wimbledon finals.

Big first serves on critical points figure to be an important factor on the rubberized asphalt courts here, as on grass at Wimbledon. Borg an average server a couple of eyacs ago, has developed a truly intimidating service for the moments when he needs it - a massive technical improvement for which he gives full credit to Bergelin.

"Jeemy's serving pretty good, but he's got to stay low on his forehand. That's the shot he's got to improve, the one that worries me," said Segura, this time out of Connor's earshot. "For some reason, he's got in the habit of coming up on it. I keep telling him, he's got to stay low on his follow-through."

Segura is a talker by nature. Also a compulsive diagrammer. A keen tactician and student of the game, he is fond of scribbling strategic points on napkins, tablecloths and scraps of paper. He loves to expound his theories. As a coach, he is an extrovert.

Bergelin, by contrast, is more subtle, almost as stolid as Borg himself. He watches his man's practice sessions - which are emotionless, but just as purposeful and uncompromising as his matches - but says little to him. He also downplays Borg's chances, at least publicly, instead of inflating them.

"This surface is something new for Bjorn, a new idea again to learn," he said. "We don't know how it will be, but we practice very hard. Now we see." When in doubt, Bergelin reckons, it's best to minimize the pressure of great expectations.

Borg played the opening match in the tennis complex on the site of the 1964 World's Fair, routing Bob Hewitt - a masterful doubles player, but too slow afoot and light of shot to be much of a singles threat at age 38.

Of the 128 men in the Open, he was just about the ideal first-round opponent for Borg.

Like most players, Borg prefers to play in daylight rather than under lights, but he agreed to play the opener - an honor Connors earlier had declined.

"They said that all the top guys would have to play once at night," said Bergelin, illuminating Borg's eagerness. "If we have to play under lights, I am happy to play Hewitt. Not so tough to see the serve, you know."

No dummy, this Bergelin. He speaks sparingly, but he knows all the angles and sees to it that Borg always plays on the house percentages. Lee voluble than many coaches about his role, Bergelin may just be the shrewdest of them all.

Englishman Fred Perry, who until Borg duplicated his feat in July was the only man since World War I to have won the Wimbledon singles three successive years, was musing recently on the subject of contemporary tennis players and their coaches:

"It's not a question anymore of whether Borg can beat Connors or (Guillermo) Vilas. It's whether Borg and Bergalin can beat Vonnors and Segura or Vilas and (Ion) Tiriac," he said. "This used to be a game of individuals, but now it's a team effort between entourages - players and their coaches, managers, gurus and what have you."

Borg has the affable and thoroughly dedicated Bergelin, the Swedish Davis Cup captain and onetime Wimbledon quarterfinalist.

Connors has his mother, Gloria, at home - he says she can give him remedial work by phone, so familiar is she with his game - and friend Lornie Kuhle as his constant "walking around guy."

Segura, the old pro and strategist who was with him in 1974, the year he won 99 of 103 matches and swept the Australian, Wimbledon and U.S. titles, has been brought back as coach for the Open.

Vilas, the defending champion here, has the massive Tiriac, patriarch of Romanian tennis. Somber, menacing, but indisputably acute, he was the driving force who whipped Vilas into reaching his potential last year, when he took the French and U.S. Open titles and amassed a 50-match winning streck.

Six American players - Harold Solomon, Roscoe Tanner, Brian Gottfried, Stan Smith, Dick Stockton and Bob Lutz - have hired Dennis Ralston, their former Davis Cup captain, to coach and train them on the international circuit.

And Vitas Geruaitis, striving to join Borg and Connors in what he calls "The Two-Mile High Clug," recently engaged Austrian Fred Stolle, his coach with the World Team Tennis New York Apples and a great playing rival of Ralston's in the mid 1960s, as his personal touring mentor.

"If you're a young player with potential to get to the top, you're crazy in this day and age not to proceed with a coach," said Arthur Ashe, who reached the pinnacle without one.

"A coach can be of inestimable value to a player, technically, tactically and as an enforcer of a training regimen," Ashe said.

"Self-discipline doesn't always work. You're better off if you've got somebody there saying, 'Look, this morning you're doing 20 minutes of forehand cross-courts, and no cheating on the last five minutes . . . then you're going to go to bed at 11:45 No shooting the breeze until 12:15.'"

Ralston and Stolle primarily are coaches, working with their players on shotmaking mechanics, tactics and conditioning. Bergelin, Tiriac and Conors' revolving retinue are much more - psychologists, traveling secretaries, surrogate fathers, confidants and "handlers" in the best tradition of the savvy prizefight managers.

They are Angelo Dundees in warmup suits.

Bergelin - a balding, rawboned man known as "Kojak" in the tennis community - is in many ways the prototype of the breed. He has been with Borg, now 22, for nine years. First he was tutor, and now he is road manager and shield against a prying, demanding world.

"He is like a big brother for me," Borg said. "We've been doing so much together - I know him so well, he knows me so well, we are very close. He's such a big help to me because he's been playing tennis himself. He knows exactly what's going on.

'It's such a big help to have someone who understands you on the court and off the court, "Borg said.

"At tournaments he takes all the telephone calls and makes the arrangemants for practice courts, partners, rackets, transportation, all these things. That's something that is unbelievably tough, because so many people call every day. He takes every call to his room. If I have to do all these things myself, I would be so tired I could not play well."

According to Bergelin, "If you are a good youngster wanting to succeed, I think it's very important to have somebody behind you who you can trust - in everything.

"Before, up to maybe two years ago, Bjorn and I were talking quite much about backhands and forehands and all kind of things like that. Today he is a champion. He knows the game. My job is different," Bergelin said.

"The biggest thing now is to see that nothing bothers him, so that he goes in the court with a clear mind."