Until John McEnroe lost in the fourth round, the biggest upset at the U.S. Open was Ivan Lendl's smile. Once as grim as the reaper, he was now all smiles. Big toothy gaping smiles.

Understandably, there were none Sunday after he lost the championship so many expected him to win, and lost it so badly, 6-3, 6-7 (2-7), 7-5, 6-0, to Jimmy Connors. But the new Lendl still was in evidence.

After Connors lost in the fourth round of Wimbledon this year, he left the hallowed grounds in a huff and a sweat, without showering, without saying a word. He was not fined for his transgression, as the rules demand, because Wimbledon decreed that certain things are understandable for losers.

After Lendl lost yesterday in front of a hostile New York crowd that has claimed Connors as its own, he came to the press conference, where once he might have faced an equally hostile reception. He offered no excuses. He had double-faulted on set point in the third and buried his head against the padded wall at courtside in pain and dismay. "If you make a mistake like that, you do not deserve to win," he said.

Lendl's performance on the court may not have earned him many friends among the tough New York crowd that cheered his self-destruction. But his performance afterward should.

Lendl, 23, never has won a Grand Slam event. Later, Connors who has won eight Grand Slam singles titles, including five U.S. Open singles titles, was asked what words of comfort he would offer Lendl. "He's still a young guy and I'm sure he's going to be around for a long time to come and have a lot more chances to win major tournaments," Connors said. " . . . I don't think it's fair to give him the title of 'choke man.' "

When reporters asked Lendl what he thought people would say about him if he finally won, he laughed and said he had confidence they would find something new to ask.

Lendl will have to contend with the same questions for a while more, but surely the most important ones are those he is asking himself. This tournament may well be remembered as the U.S. Psychological Open. When Martina Navratilova beat Chris Evert Lloyd, 6-1, 6-3, and finally won her first U.S. Open on Saturday, after 11 years of trying, she said exultantly, "It proves there is no jinx."

What it proved was that her talent had finally and completely subdued self-doubt. Her toughest win? "Mentally, yes," she said. "It wasn't as hard physically as winning Wimbledon the first time . . . Now I'm almost 27 and I've worked hard to get here and you realize how much more it means . . . I know my time is running out. I see the sand trickling down. It means a whole lot. It is the most satisfying of all."

When he examines what happened to him on Sunday, Lendl may take heart from Navratilova. In his case, his talent was subdued by doubt. But in many ways, Lendl is going through the same transformation that Navratilova began when she defected from Czechoslovakia during the 1975 Open.

He is making an effort to fit in. Questions that once might have made him snarl were parried last week with good humor and charm. "He is growing up," Wotjek Fibak, his friend and manager, said.

"One more inch," Lendl said, playfully.

"He is more mature, more tolerant," said Fibak. "He says people are quoting him exactly the way he is quoting himself. Also he sees a good image helps him in marketing, that it's not very smart to fight with the press."

"I always knew it, I just didn't feel comfortable," Lendl said. "Probably I just grew up. Before, tennis was the only thing I was looking for. Now it's not the only thing."

Fibak said has noticed the change for a few months. Jerry Solomon, Lendl's representative at ProServ, saw it "basically between the French (Open) and Wimbledon. When he didn't win the French it had a big impact . . . He's more trusting. He sees that Americans are that way."

Though Fibak and Solomon had been talking to Lendl about these things for a long time, they sat down and had a conversation a couple of weeks ago in Montreal and laid it on the line. "It's what I'd characterize as a pretty tough discussion on where he's going," Solomon said, "It had an affect."

In certain moments, certain events like a tennis match become a crucible. A man battles not just his opponent but the stresses within himself. That can be the harder loss.

If Lendl confronts that truth, and continues to address it as forthrightly as he did after losing that match Sunday, he may find a lot of people rooting for him to win.