Once an athlete reaches the top level of his game, it often seems that his central project suddenly becomes more complicated. Instead of learning to improve his game, he must learn to improve himself.

Where victory was once the only goal, the new obsession becomes a search for grace and personal style.

This is the joker in the deck, the twist at the end of the short story. Nobody tells the John McEnroes and Ivan Lendls that, after they've become stars, they must still become men who can carry themselves well in their own and in others' eyes.

At the U.S. Open today, both McEnroe, who won his national title for the fourth time, and Lendl, who lost in the final for the third straight year, seemed to make some tiny, arduous and incremental progress in their battles to change their personalities.

Neither one yet seems terribly happy with himself. Both talk constantly about their attempts at self improvement. And both still agonize that they are barely popular, rarely appreciated and seldom cheered.

Nonetheless, both seemed more at ease this evening, McEnroe with his customary triumph in a Grand Slam setting and Lendl with his customary humiliation.

Neither needs to learn much more about tennis. They stand Nos. 1 and 2 in career money winnings in history with $12 million between them. What has tormented both in recent years is the thought that, while they may be winning at tennis, they may be losing at life.

Tennis often seems to be the sport of young unhappy millionaires with McEnroe and Lendl the two saddest male examples of the phenomenon.

McNasty and Ivan the Choker. What gruesome nicknames for fellows who are not felons. In both their cases, great talent, dedication and wealth have all been tinged with bitterness because of a flaw of temperament or, perhaps, merely by a set of mannerisms that run against the grain of contemporary social rules.

Some say that Lendl should work on his volley and his backhand. In his heart, what he really wishes he could improve are his smile and his snarl. Nobody thinks he has humor or heart.

After being demolished in straight sets, 6-3, 6-4, 6-1, Lendl was as glum as, among tennis players, only he knows how to be. When he was asked if he thought he was in danger of becoming "the heir to the ghost of Borg (who never won the Open in 10 tries)," Lendl replied, "You just gave me one ghost (i.e., can't win a Slam title) a few years ago. Now you are giving me a new one. Exactly what I was expecting."

Despite this anguish, Lendl did not seem a fraction as demoralized here as he did last year after quitting in a 6-0 final set against Jimmy Connors.

"He was just playing well. All I could do was just keep trying and I did," said Lendl, who gave a credible effort to the end, despite a 225-minute, five-set match the day before.

"The only realistic change I can make in my strategy against him is to return his serve better. Without breaking him, you will never beat him. I just have to practice against left-handers and return and return and return. The problem is that, no matter what left-hander you find in the world, none of them is going to serve and volley like McEnroe."

Just as Lendl knows he must show more emotion, as he did in beating Cash, he also knows he must vary his strategy against McEnroe by coming to the net more often. But playing styles are linked to personality styles and they're tough to change.

"He was coming in a lot early in the match. He surprised me. But after he got behind, he gave up on that strategy and just looked kind of discouraged," said McEnroe.

Sometimes an athlete's progress is so slow that you wonder if it exists at all. Was Lendl's fire against Cash a mirage? Was that discouraged look this afternoon the real Lendl returning?

If the jury is still out on Lendl's gumption and his ability to incorporate new tactics into a stiff-necked personality, then the first hints of a verdict may be arriving on McEnroe.

Maybe it's wishful thinking, maybe it's just the 89th New Year's resolution by a spoiled child, but it seems McEnroe is growing up. A little.

Yes, he rolled on his back today and held his hands a foot apart to show the crowd how badly he'd been robbed by an offending official. Yes, he grumped and grumbled enough to get himself a punch in the nose in a public park match. Yes, he screamed one four-letter word.

Archivists date McEnroe's first vow of improved behavior as June 1977.

Despite all this, McEnroe sure talks a better game these days.

"I think I've gotten better this year . . . and part of the reason is that, after losing the French, I had to stay away from controversy. It was taking too much away from my game. I said to myself, 'I just gotta stop doing it.' "

In the first dozen games this day, when he was frazzled and short-fused from Saturday's effort against Jimmy Connors, McEnroe could have blown his cork and blown a title, too. Instead, "I was never close to losing it."

Minutes later, McEnroe drew press conference double takes when, out of the blue, he offered, "That's what life is all about: compromises . . . Life is not fun banging your head against a brick wall."

For many, this day may go down as a complete bore, relieved only by its briefness. If Saturday's 12 hours of tension was one of the best tennis marathons in years, then today's 100 minutes of mistakes was exhausted stuff. If you went to the beach and taped this mess on your VCR for later consumption, don't even bother to hit "rewind."

For those, however, who would just as soon watch two decent young men grow up as watch them play a great tennis match, then perhaps this day wasn't such a waste after all.