A crowd of 18,753 came to Madison Square Garden tonight hoping to see a fight between John McEnroe and Jimmy Connors.

Instead, a tennis match broke out.

Manhattan's jet set glamor troops were out in force, anticipating an unsanctioned middleweight title bout between the 5-foot-10, 155-pound Connors and the 5-11, 165-pound McEnroe--the pair who had exchanged curses, pointed fingers and light shoves in Rosemont, Ill., on Sunday.

Much to the chagrin of this congregation, including everybody from Hank Aaron to Liza Minnelli, McEnroe won a swift, simple and tactically surprising straight-set match from Connors, 6-2, 7-5, in the second round of the Volvo Masters.

"I played it pretty smart," said the uncharacteristically patient and almost conservative McEnroe, who had lost his last two meetings with Connors. "Before, I got careless and lost.

"When it got close, I always came to the net too quickly. Jimmy'd pass me a couple of times and get pumped up," added McEnroe, who saved his numerous temper outbursts this evening to berating and insulting officials. "Tonight, I made Jimmy earn every point."

"He may have to do it again before this week is out," vowed Connors, referring to the distinct possibility that these same two may be meeting in Sunday's final with $100,000 to the champion. "I'm not dead yet . . . I'm just shot."

McEnroe's second consecutive victory in this eight-man $400,000 round-robin event assured him of a place in Saturday's semifinals. In other matches yesterday, Ivan Lendl, also 2-0 in matches, clinched a place in the semis by beating Guillermo Vilas, 6-4, 6-1; Elliot Teltscher evened his record at 1-1 by beating Roscoe Tanner, 4-6, 6-1, 6-4, and Vitas Gerulaitis defeated Jose-Luis Clerc, 7-6 (7-5), 6-1.

In Friday's afternoon matches, McEnroe meets Teltscher and Lendl faces Clerc in the sort of relatively meaningless matches that, in the past, were said to be played in such a way as to either conserve energy or created a preferred matchup in the semis.

This year, a $30,000 bonus--instead of last year's $10,000--goes to each "group leader" from the first three rounds; McEnroe and Lendl could grab that cash with victories in their respective halves of the field.

In Friday night's significant semis-at-stake matches, Connors, who is 1-1, will face Tanner (0-2) and Vilas (1-1) takes on Gerulaitis.

This evening's McEnroe-Connors affair, easily the highlight of this Masters so far, was a fascinating battle of tactics. Usually, as on Sunday, Connors slugs from the base line, trying to create openings to come to the net behind his sharply angled, go-for-the-lines ground strokes and deep approaches. McEnroe plays like Mack the Knife with his repertoire of snaking serves, amazing volleys and overheads.

This time, however, McEnroe scratched that plan. Partly, it was a result of a nagging left hamstring pull that left McEnroe bandaged and feeling "tight" and restricted in his usually superhuman volleying. Also, McEnroe had observed his own impatience on Sunday, his desire to punch out Connors with big, humiliating putaways on crucial points.

So, McEnroe counseled himself in the one virtue most thought he lacked: patience.

"I kept him off balance and waited for my spots to come to the net," said McEnroe who, starting at 2-2 in the first set won seven consecutive games in which he allowed Connors only eight points.

This was hardly an evenly fought match. McEnroe's speed and spin-changing didn't permit Connors to get a confident sense of pace; as the match progressed, Connors' usually reliable ground strokes deteriorated.

Finally, in the final game of the match, McEnroe got three match points at 0-40 on Connors' serve and, after one Jimbo point, finished off his tormenter with a buzzing forehand pass down the line.

The dominant undercurrent of this 105-minute match was not hostility between McEnroe and Connors, who have had on-court, oneupmanship tiffs before, but actually are fairly close to being friends. Instead, it was the complete contempt in which both players held the linespeople and umpire--the expense money-only amateurs who sometimes seem underqualified and out of place in such big-time events.

The umpire gave McEnroe a conduct warning for aimlessly swatting a ball in disgust and also gave him a time-delay warning for his protracted whining at another juncture. He failed, however, to see McEnroe kick the box surrounding a linesman; the box hit the linesman, the linesman went to beg the ump for protection from the miscreant and the ump did nothing.

In particular, McEnroe raged at one lineswoman, screaming that she was a cheat and even tossing the ball in the air as though about to smash it at her from six feet away. She merely turned her head away. "I've had trouble with that lady since I was 12 years old," said McEnroe.

And, since McEnroe's conduct hasn't changed since then, he still does.

For his part, Connors screamed at the ump, "I want you out of the chair."

What would happen, Connors was asked, if he and McEnroe were allowed to call their own lines.

"We'd probably kill each other," said Connors.