With a display of true grit and self-control that indicated he has grown up the last four months while emerging as one of the world's foremost tennis players, 19-year-old John McEnroe beat Arthur Ashe, the people's choice, today in a stiring, absorbing climax to the Colgate Grand Prix Masters Turnament.

McEnroe also benefited from a bit of luck -- a questionable call on the second match point against him -- but that should not detract from what many observers saw as his graduation from boy to man, from brat to proven professional.

He guttily came back from 1-4 in the final set, stared down two match points and the will of the majority at 4-5, and won this splendid, 2 1/2-hour, emotional wringer of a final, 6-7, 6-3, 7-5.

To do so, McEnroe blocked out the dispiriting, potentially paralyzing effects of a Madison Square Garden crowd so fiercely pro-Ashe that it practically lifted the comebacking, 35-year-old former World No. I from 3-5 down to a tie-breaker victory in the first set.

He kept his poise and composure despite some maddeningly close line calls, most of which went against him. And even though all of the 17,000 spectators, except his parents and a few friends and neighbors from nearby Douglaston, N.Y., seemed hostile, he never lapsed into the puerile behavior that got him branded early as a Peck's Bad Boy, a reputation he seems intent on shedding.

McEnroe adjusted smartly to Ashe's clever, thoughful game plan -- based on taking away his sliced lefty serve deep to Ashe's right-handed back-hand-and ultimately won on a day he didn't serve well.

Pancho Gonzales used to say that a champion was distinguished by his ability to win when he didn't have his best "stuff." If that is true, McEnroe -- who got only 60 percent of his usually telling first serves i court -- can call himself a champion today.

This victory was the biggest to date for McEnroe, the youngest player since Ken Rosewald in the 1950s to have reached the semifinals of both Wimbledon (1977) and the U.S. Open (1978).It capped a spectacular four-month spree during which he has won five tournaments in singles, eight in doubles, and led the U.S. to its first victory in the Davis Cup finals since 1972.

The $100,000 he collected today, along with the $20,000 he won for partnering Peter Fleming to the Masters doubles title over Wojtek Fibak and Tom Okker Saturday night, increased the prize money McEnroe has grabbed since turning pro last June 10 to $463,866.

More important, it legitimized in the eyes of tennis insiders his admission to the highest rank of tennis stars, the penthouse suite which Vitas Gerulaitis longingly calls "the Two-Mile High Club."

Even though the absence of Wimbledon champ Borg, Guillermo Vilas and Gerulaitis, plus the default of defending champion Jimmy Connors to McEnroe on Thursday, diminished the importance of the Masters -- a $400,000 playoff intended to bring together the top eight finishers in the season-long Colgare Grand Prix tournament series -- the final provided the sort of big-match crucible McEnroe had not really experienced before.

Ashe, the 1975 Wimbledon and world champion who has climbed back to No. 13 in the world rankings after heel surgery that sidelined him most of 1977, put him to the acid test. McEnroe showed that under intimidating pressure, he has a winner's guts and nerve to go with his immense talent.

"This situation called for a special sort of toughness, and I think today he met the challenge," said Ashe, who received $64,000 as runner-up. "This victory goves him a lot of credibility. Definitely."

Ashe finished 10th in the 1978 Grand Prix standings, but got into the Masters when Brog and Vilas declined. He lost to McEnroe, beat Harold Solomon and got a default over the infured Connors to finish 2-1 in the roundrobin portion of the tournament, gaining a spot in the semifinals, and found his vintage serve and backhand to beat Brian Gottfried Saturday and earn a return bout with McEnroe.

After his 6-3, 6-1 loss to McEnroe Wednesday evening, Ashe devised a new strategy for the rematch.

Receiving servce, he gave McEnroe the forehandside -- especially in the left court, where he stood behind the doubles alley, guarding against the deep slice that slides low and menacing off the quick synthetic court here, snaking away from a right-hander.

"He more or less took away the wide serve. If I had been serving well, I could have aced him down the middle all day, but it's hard for a lefty to hit that flat serve to the forehand anyway, and I was missing it," said McEnroe, who had lost his serve only once in beating Ashe, Connors (7-5, 3-0, retired), Solomon and Eddie Dibbs in straight sets. "I wasn't serving as hard or well as I had been, and that gurt me. It took a lot more energy to hold serve."

McEnroe led, 5-3, 40-0, triple set point, but double-faulted three points in a row, the crowd cheering more loudly on each of them. When Ashe broke after saving a fourth set point, they roared. And when he held serve at love to 6-5, and won a best-of-12-point tie breaker, 7-5, with a service winner to the backhand, they went absloutely crazy.

Ashe had advantage on McEnroe's serve at 1-1 in the second set, but from that point, "Junior" reeled off 12 of 13 points and raced away to 5-2, then served his way out of a 15-40 jam at 5-3.

McEnroe saved two break points after double-faulting to 15-40 in the second game of the final set. But after Ashe held at love with two of his nine aces, he lost his serve on another of his total of seven double faults.

Ashe, crackling now, held with another ace to 4-1. To win, he merely had to "tend to my knitting," as he puts it -- to hold his serve twice. But at 4-2, he lost it, missing six of eight first serves, including the last four. On the second break point, McEnroe got him with a buzzing foregand cross-court passing shot.

Ashe held at love to 5-4, and got to 15-40 on McEnroe's serve -- double match point -- with a couple of delicate chips and a down-the-line return winner of a good second serve to his backhand corner.

"Come on, Arthur," screamed thousands of spectators, but Ashe netted a backhand down-the-line pass. At 30-40, McEnroe served, Ashe laced a backhand return down the line that forced a backhand cress-court volley error -- but there was a late "fault" call.

"I'd bet all the money I have in the world that the first serve was good," Ashe said forlornly afterward, "but that's ancient history."

McEnroe's secend serve surprised Ashe by coming to his forehand, and he knocked his return into the net. "I was choking -- I just didn't want to double-fault the match away," McEnroe admitted candidly, "but he couldn't believe the lousy serve and rushed himself. It was kind of an allaround had point."

Ashe had 40-15 and one advantage -- three game points in all -- as he served at 5-5. But McEnroe creamed an overhead after getting in behind an offensive lob, a tactic Ashe had used successfully earlier. Then Ashe pushed a forehand volley long, and McEnroe buzzed a forcing return and a topspin backhand cross-court pass on the run for the match.

"I don't know how I won, I haven't figured it out yet," McEnroe gasped later, still startled that he had over-come Ashe's guile.When he thinks about it, the answer will come to him: quickness, skill, true grit, and composure under intense pressure.