NEW YORK, SEPT. 4 -- See John McEnroe while you still can, jutting his chin at a spectator, hitting a reflex volley, picking at his shirt or pulling at his pocket. "I'm not going to be around much more, let's face it," he said. He comes and he goes, a little more faded with every appearance, like some kind of dimming sprite. But for the moment he is very definitely here, and he has the U.S. Open by the throat.

It's as if the applause has brought him back, at 31, on yet another painstaking comeback, unseeded for the first time in 13 years here but in the quarterfinals of his hometown tournament he has not won since 1984. McEnroe has beaten two seeded players in the last two rounds. He meets unseeded David Wheaton Wednesday night with an excellent chance of making the semifinals, and there is the growing sense that on a given night in this gritty, run-down borough of Queens, where he is alternately revered and jeered, he just might be capable of winning again.

"That's why I'm doing it," he said. "If I didn't think that it would happen I wouldn't have done it in the first place."

In the last week McEnroe has defeated 10th-seeded Andrei Chesnokov, 6-3, 7-5, 6-4, to reach the round of 16. Then he survived five sets with seventh-seeded Emilio Sanchez, rallying, 7-6 (8-6), 3-6, 4-6, 6-4, 6-3, to make the quarterfinals of the Open for the first time since 1987.

What are his chances? "It's ridiculous to even think about that," he said after the first round. After he beat Chesnokov, the Soviet said poutily, "I think he would have to play ten times better." Still a few days later, with a semifinal meeting with old nemesis Ivan Lendl a strong possibility, McEnroe said, "It would be nice to be able to get that chance, to be able to play another big match on a big occasion."

That possibility has taken over the Open. New York Mayor David Dinkins watches McEnroe intently, so does actor Tom Cruise, so does Jennifer Capriati. Martina Navratilova predicted he would win the Chesnokov match "just because of the crowd." Andre Agassi, seeded fourth, said, "I'd love to see him in the finals." So, apparently, would second-seeded Boris Becker, who watched the 4-hour 20-minute match against Sanchez. "How could you miss it?" Becker said. "It was so long." Becker also revealed that he and McEnroe had a long, rather intimate talk last week, implying his encouragement. "We had a visit," he said. "It was private."

Sometimes McEnroe has played loosely, but he has played mainly evocatively, riding this unfamiliar sensation from the crowds and colleagues of being actually sort of beloved. There are still detractors, like the woman who heckled him from a courtside seat during the Chesnokov match, the two of them jawing between points. But perhaps the most surprising thing about McEnroe's progress through the tournament has been the warmth with which the nearby Douglaston, N.Y., native and four-time champion has been received.

"I'm very brash in my attitude so it was difficult for people to easily latch on," he said. "Now it's more easy. Being the underdog, not having been successful over the last years, all the combinations have started to gel."

The poor unfortunate who must wade into the middle of all this sentiment is Wheaton, 21, of Lake Minnetonka, Minn., the home town as world champion figure skater Jill Trenary and Tour de France victor Greg LeMond. Wheaton is a fresh-faced serve-and-volleyer of 6 feet 4 who has quietly progressed through the tournament largely aided by the first-round loss of top-seeded Stefan Edberg. He played his first-round match on an outer court at 9:45 one night.

"There were about seven people there," he said. "And six of them were my family."

When he and McEnroe meet in Wednesday's featured evening match, it will mark Wheaton's first singles match on Stadium Court of the National Tennis Center. He is cast as the ingenue and a potential villain. He is braced for the crowd. "It's an added detail you have to shut out, like if it was windy or something," he said. "I won't be counting heads." A composed player with neat strokes who is ranked No. 44, he resists the idea he will be part of an extraordinary occasion.

"It's just a tennis match," he said. "He might beat me in straight sets, or it may be a great match. There's no reason to get excited until we see the results."

Wheaton should not be underestimated. He is not unfamiliar with big matches, reaching the quarterfinals of this year's Australian Open. He is among the new breed of big servers who points out that he may have the surprise factor on his side. "I've seen him play a lot more than he's seen me," Wheaton said of McEnroe.

The same might be said of Pete Sampras, the 19-year-old serve-and-volleyer who is third-seeded Lendl's quarterfinal opponent, a rising star already ranked No. 12. "I'm going to swing away," he said. McEnroe has 75 career titles, Wheaton one. Lendl has 87, Sampras two. Wheaton's only previous encounters with McEnroe came in a doubles match in Australia last year, and as a child when he watched him on television play Wimbledon. Sampras and Lendl have never met.

McEnroe is taking nothing for granted. The emotion with which he has played here hides the fact that his game is still under repair, a long-term rehabilitation project. He let it slip after he was disqualified from the Australian Open in January and fined $6,500 for an ugly tantrum. Sulking, he fell into several months of idleness, plunging from No. 4 to No. 20 in the rankings. He lazily readied himself for Wimbledon by practicing an hour a day with Vitas Gerulaitis, was beaten in the first round, and went home saying that he's "going home to get my act together. And there's a lot of act to get together. I could be out of the game in six months."

McEnroe has given himself a time frame of 18 months for one last try to reach the potential that has been unfulfilled since 1985, when he fell to No. 2 after holding the top ranking for four years. He was not among the top five, or even the top 10 for much of the next three years, as he grappled with a transformation into a family man with his marriage to actress Tatum O'Neal and the birth of two sons. Finally, he rose to No. 4 last season and made the Wimbledon semifinals, but was beaten in the U.S. Open's second round by obscure Paul Haarhuis.

McEnroe comes and goes like that because his game was natural and when it went he did not know how to find it again. A dedication to training and discipline were never among his traits. Rather, he was confused and ambivalent, undecided about exactly how much effort to put forth, and unwilling to revert to his old monstrous tempers.

"As you get older your responsibilities and priorities change," he said. It's a different thing, you want to improve yourself as a person, it's not only if you can win on the court, that's not important to me at a certain point, winning at any cost. . . . I don't think any player wants their whole being based on whether they win a match."

This attempt, which McEnroe says will be his absolute last, somehow seems different. For the first time, he is considering seeking a coach. He has regrouped by playing tournaments like this, raising his game almost on memory or habit. When the current effort ends, he will retire, and would like to do so knowing he gave it, finally, his best effort so "you know that you're not cheating yourself and you're not cheating your fans and the spirit of tennis. I don't want to play that way anymore." How successful this latest transformation is will be seen on a late-summer evening in a capacity stadium.