It is a measure of the stature, intensity, and mutual respect inherent in the current rivalry between Bjorn Borg and John McEnroe that when they duel across a net, even the quintessentially stolid Borg can get uptight under the pressure. When the two best tennis players in the world collide, every point is crucial. So important, we know now, that even Borg will wrangle with an umpire for what he believes to be rightfully his.

Borg, the five-time Wimbledon and French Open champion, beat 1979-80 U.S. Open champion (mcEnroe, 6-4, 6-7, 7-6, in a peculiar and fascinating match that began a little after 9:30 on Thursday night and finished at 12:20 a.m. Friday at Madison Square Garden. The victory qualified Borg for Saturday's semifinals of the $400,000 Volvo Grand Prix Masters tournament, and knocked McEnroe out of the running with an 0-2 round-robin record, but it will be remembered most as The Night Borg Lost His Temper. r

When British umpire Mike Lugg overruled a linesman's call at 3-3 in the second set tie breaker, awarding to McEnroe a point that Borg was "100 percent sure" he had won with a blistering forehand passing shot, the Swede briefly blew his legendary cool. He decided that he was mad as hell and wasn't going to take it anymore. He protested for a full four minutes, and was docked two penalty points by Lugg for delay of game, the first time he has been so penalized in his remarkable career.

Think about it. Borg getting disciplined for arguing a line call is about as unlikely as Ronald Reagan nominating Frank Church for Secretary of State.

Clint Eastwood, Robert Duvall, Benny Goodman, and the rest of the celebrities in the capacity crowd of 19,103 spectators were astonished. The many great players of the past and present who were at courtside, from Jack Kramer and Billie Jean King to Don Budge and Tracy Austin, shook their heads incredulously. Even McEnroe was agog.

"I couldn't believe it happened. I was totally shocked," said Borg's chief antagonist for professional supremacy. "It was so unusual to see something like that happen with Bjorn. I didn't know how to react. I didn't feel right about taking the points. I wanted to play it from 4-3 in the tie breaker. I didn't know whether to tank the match, or try, or what to do."

True to his splendidly combative nature, McEnroe tried like a son of a gun when the match resumed with Borg set point down in the tie breaker. He forgot about the pulled hamstring muscle for which he had his left thigh heavily taped, and challenged Borg shot for shot in a tense, gripping final set.

Borg, still angry long after losing the second set tie breaker, 7-3, channeled his fury into on-court aggression. He served mightily in the third set, and went to the net more relentlessly than he had earlier. There were no service breaks in the set, and when the match went into another tie breaker, Borg was all business.

At 1-1, he won a marvelous point. He ran around a second serve and hit a punishing forehand return, but McEnroe lunged for the ball and coaxed over a forehand drop volley that looked like a winner. Borg was on the ball swiftly, though, and looped a topspin forehand passing shot down the line.

McEnroe got back to 2-3 with a scorching backhand passing shot, but then Borg whacked a backhand cross-court return winner off a good first serve for 4-2. McEnroe dove for the ball in vain, sprawled face down on the court, and pounded his fist in frustration. He didn't win another point, as Borg ran out the tie breaker, 7-2.

Borg and McEnroe have played five times in the past 12 months, and four of the matches have been gems. In the semifinals of last year's Masters, Borg won, 6-7, 6-3, 7-6. In a majestic Wimbledon final, Borg won, 8-6, in the fifth set, after McEnroe saved several match points in the fourth, including five in a 34-point, 22-minute, tie breaker to end all tie breakers. McEnroe got revenge in the U.S. Open final, 6-4, in the fifth set, after Borg somehow came from two sets down on a day when he was far below his best.

Thursday night's match was not the best they have played -- it had its sublime moments, but there were inconsistent patches as well -- but it was surely the most bizarre, and another dramatic masterpiece. McEnroe, the sport's angry young man, peeved the majority of the SRO crowd with temperamental outbursts and bitterly self-critical asides, even as he was gaining their grudging admiration for his feisty fighting spirit, guts, and dazzling play at the net.

The spectators loathed McEnroe the spoiled brat, but loved McEnroe the scrapper who dug in and broke Borg when he served for the match at 6-5 in the second set. And they gave him a standing ovation in the first game of the third set when he gave away a point, intentionally hitting a return of serve 15 rows into the stands after Borg was victimized by another line call that both players knew was a mistake.

Think of it! McEnroe giving Borg a point to "even up" a bad call seems as unlikely as Borg making a scene. Next thing you know, they'll be telling us that Ronald Reagan plays left wing.