A year ago, John McEnroe supplanted Bjorn Borg as the No. 1 player in the world. Suddenly, he finds, "I can't buy a point when I really need it."

McEnroe, who is seeded No. 1 at the U.S. Open, being played at the National Tennis Center, hasn't won a significant event since the U.S. Pro Indoor in Philadelphia in January.

"I tell myself not to panic, but I'm worried," he says with characteristic candor.

Slumps happen to all athletes. But how McEnroe ends his will determine whether he reemerges as the confident player he once was. And whether he remains the No. 1-ranked player in the world.

No two athletes regain confidence the same way. For McEnroe, the way back will be foreign because it calls for the one thing never before required of him: hard work.

McEnroe has never worked hard before because he hasn't had to. He is a natural athlete, and thus "feel" for the play and pace has come naturally. Now he has to reason his way out of situations that once were solved by instinct.

He has a rather severe blister on his right foot. This has developed because he has been playing on soft courts -- grass and clay -- or resting for five months. The Open is played on Decoturf II, a hard court, so it can be expected that he will be bothered by the blister here.

But McEnroe's most severe problem is his self-esteem. Athletes, and in particular young athletes, equate their self-worth with their performance. The feeling seems to be, "If I'm not playing well, I'm not worth a damn."

As McEnroe's Davis Cup captain, I try to tell him that slumps happen to everybody and that they are seldom as grave as imagined. I know that a victory here at the Open could convince him -- and turn his game around.

Although McEnroe is seeded No. 1, seven of 10 tennis writers here don't think he will win, picking instead Jimmy Connors (who beat him in the Wimbledon final this year) or Ivan Lendl. But he retains the seeding on the basis of the ATP computer rankings, which have him No. 1.

In this tournament, he has three potentially difficult opponents in his half of the 128-man draw: Gene Mayer, Lendl and Mats Wilander. Connors, seeded second, is in the other half of the draw and Vitas Gerulaitis was eliminated today in the first round, losing to Fritz Buehning, 6-4, 7-6 (7-4), 6-3.

Thursday, McEnroe will finish a first round match with Tim Gullikson that was suspended tonight with the score 3-3 in the first set. His first big test should come in the round of eight against Mayer, who has just the sort of error-free approach that could rattle a tentative player.

McEnroe may feel that he must play exceptionally well to beat Mayer now. He's right. If he does, he's likely to face Lendl or Wilander in the semifinals. Lendl thinks he's got McEnroe's number, and since he has beaten him the last five times they've played, maybe he's right. But if McEnroe gets to the semis, he should have back some confidence.

If Wilander is to reach the semis, he will have to beat Lendl. In a recent Davis Cup thriller, McEnroe beat Wilander in five sets, advancing the U.S. to the semifinals of the Cup.

The other half of the draw is Connors' to lose. But it's silly to talk about a U.S. Open final for McEnroe when he says he "can't buy a point."

He is as intense an athlete as I have met. On the court, he believes he must fight for what he believes to be true. But that's working against him because, with increasing frequency, what he believes to be true and what is actually true are not the same.

Peter Fleming, his good friend and doubles partner, and lifelong pal Peter Rennert have McEnroe's predicament in focus. "John likes being ranked No. 1, but can't handle all the crap that goes with it," says Fleming. To which Rennert adds, "Yeah, but he's so talented that it won't matter, anyway."

Fleming is right; Rennert is only half-right. McEnroe is beginning to see what he believes instead of what is real. And if that continues, talent is not enough.