John McEnroe's complexion was as red from running around today in the muggy, 95-degree sunshine as a lobster's gets from diving into boiling water after he beat old acquaintance Rick Meyer, 6-1, 6-1, 4-6, 6-2, in the third round of the U.S. Open tennis championships.

McEnroe's flushed cheeks may also have reflected his aggravation with some of the spectators in the packed grandstand at the National Tennis Center, who had loudly and rudely cheered his double faults and errors. He exchanged unpleasant words with several of them.

"I just think it's wrong. When I'm double faulting, I don't think there's any reason they should be clapping. I just don't settle for that," said the volatile defending champion from nearby Douglaston.

Some of those who were cheering against McEnroe were friends of Meyer, 24, another Long Islander. They were clustered in a courtside box from which they shouted encouragement to Meyer. There were about 10 of them, but in their midst was a McEnroe supporter.

"Well, one out of 10 is better than none out of 10," McEnroe observed latter, with a positively wicked grin.

McEnroe had hoped that his five-set loss in this summer's all-time Wimbledon final, in which he behaved impeccably and played his heart out before losing to Bjorn Borg, would change the public's attitude toward him. He hoped people would forget his petulant tantrum's of the past and realize that at age 21 he is maturing.

To an extent his hopes have been realized. But there are still hecklers, and he still has rabbit ears and a short fuse -- though not as short as it use to be. He is a realist.

"There are more people rooting for me, but New York's always going to be the same, no matter what happens," he said. "You're never going to get rid of all the bad actors. I mean, during our doubles yesterday, there were people fighting in the stands. . . . It looked pretty interesting for awhile."

Two weeks before the Open, McEnroe pulled out of the Canadian Open with a sprained left ankle. A week later, he lost in the first round of a tournament in Atlanta to John Austin, whose biggest title is the Wimbledon mixed, which he won with his little sister Tracy.

Although he didn't say it in so many words, McEnroe implied that he didn't think much of his chances for successfully defending his title when the 13-day Open began last Tuesday.

He was not moving well at the net. He was not jumping into his serves or overhead smashes. He was subconsciously favoring his gimpy ankle, and playing lazy shots.

But now he in the fourth round of the Open, paired against Frenchman Pascal Portes, and playing with more assurance each round. He may yet meet Jimmy Connors in the semifinals, Borg again in the peoples choice for a final, even defend the title he won over Long Island neighbor Vitas Gerulaitis last year.

McEnroe took three weeks off after Wimbledon, but then his injury disrupted the buildup he planned for the Open. Now he feels his form coming back, albeit slowly, gradually.

"I was favoring the ankle in Atlanta. Not as much here," he said. "I think as I play more matches on it, as I move around more, I'll get more confidence in it."

He played well for most of the match against Meyer, a University of Pennsyvania grad in accounting who used to play with McEnroe at the Port Washington Tennis Academy on Long Island, and beat him twice a few years back when the Penn varsity played a Port Washington team that included the then anonymous McEnroe.

In the first two sets, McEnroe's lefty serves were spinning off the court like nasty snakes. He let up in the third, and let Meyer back in the match, but picked up his level again and closed him out in the fourth. Overall, McEnroe was pleased.

"When you lose a set, I guess it's good in a way, if you win the tournament and look back. Like when I lost a set to Nasty last year," said McEnroe, recalling his tumultuous second round victory over the line Nastase that nearly plunged the 1979 Open into a riot, "I think I really picked up after that."

Indeed. He didn't lose another set in the tournament.

"Hopefully, it'll be the same thing again," McEnroe said. "I think each match I've gotten a little better, which is important. I can't say that I'm playing at the top of my game, but the guys I've played haven't been at the top of their games, either, so it's hard to really judge. I just feel that I'm a lot sharper than I was at the start of the tournament."