Before the women's final in the U.S. Open this morning, Chris Evert Lloyd walked into the locker room and found Hana Mandlikova eating a piece of cheesecake. "I couldn't believe it," Evert said. "I felt like asking, 'Didn't you read the newspapers?'"

Evert, who earlier in the tournament became ill after eating cheesecake, overcame Mandlikova, 6-3, 6-1, today to win her sixth U.S. Open championship, only the third woman to do so. Was Mandlikova, eating her cheesecake, merely oblivious, as she was at Wimbledon last year when she innocently asked the identity of Maureen Connolly before their match? Or was she trying to psych out the champion whose greatest strength is her psyche? "I think she needed it for energy," Evert, $90,000 richer, said, smiling.

No amount of cheesecake would have been enough to defeat Evert today. She was determined to win her first Grand Slam tournament of the year. "I was kind of in a trance," she said. "I wanted it so badly."

Evert wanted to set a record for the most matches won by a woman at the Open (66), and did. She wanted to set a record for the first woman to win a Grand Slam event nine years in a row, and did. She wanted to set a record for the most Opens won by a woman, and thought she did. "I thought the record was six," she said, shaking her head. "So now it's eight, huh (held by Molla Mallory)?"

And she wanted to set the record straight for those who were under the misguided impression that it was time to write her off, that it was time for her to retire to a genteel life of motherhood. Her husband, John Lloyd, said there were (unfounded) rumors that this week's food poisoning was really morning sickness. She would not have played under those conditions, he said.

"People were saying, 'It's a one-woman race,'" Lloyd said. "I can't understand how anyone can write her off. That's like writing Borg off. And when she gets written off, it really gets her ego going."

Evert declined to say whether she would be back next year to defend the championship. She will make those decisions in December. Mandlikova, who has now lost to her in the finals of three Grand Slam events (1980 U.S. Open, 1981 Wimbledon), said, "She said that (she will retire) after the 1981 final and I said, 'Oh, God, I am so happy.' But she is playing better and better so I think I better not believe her anymore.

"She's getting older and playing better," she added.

John Lloyd said his wife was playing at 85 percent of capacity, all that was required. More dispassionate observers gave her higher marks. Neither her serve nor her concentration were ever broken, Lloyd said.

"Before Wimbledon, she was too casual. This is the first time since Wimbledon last year that she believed from the beginning that she was going to win."

Evert made only six unforced errors, one on her backhand, to Mandlikova's 18 backhand and eight forehand errors. In the three games that Mandlikova had break points, Evert summoned enough will to deny them to her, including two in the last game of the match.

In the fourth game of the first set, ahead, 2-1, Mandlikova momentarily gained an advantage. But Evert won the next three points and the game. Mandlikova lost the next game on her serve, at 15-40, when Evert hit a forehand cross-court winner for the first break of the match.

The eighth game was the game of the match. Evert led, 4-3. Six times they went to deuce. Three times, Mandlikova had break points and made the least of them. She hung back, exchanging ground strokes with the woman who patented them, mostly because, she said, "Chris plays very long, deep balls, so it's difficult to come to the net that often."

Mandlikova won three points in the game on deep, penetrating ground strokes, one of which gave her the first break point. A strong cross-court volley that Evert could only pop up (Mandlikova put away the dinky little thing) gave her the second breaker. A forehand winner off a second serve gave her the third.

But by now, Evert was deep in her trance. Her ground strokes grew bolder, deeper, more difficult to return; a backhand deep in the corner saved the second break point. Mandlikova wound up and missed it entirely. A netted forehand volley off a better one by Evert saved the third break point.

Evert gained the upper hand in similarly unaccustomed fashion, at the net. She came in on a short ball and hit a deep forehand approach. Mandlikova tried to pass. Evert hit one forehand cross-court volley, then another. The second sufficed.

Evert broke in the next game to win the first set and broke again in the second game of the second set. By the time Evert held to make it 3-0, Mandlikova seemed resigned to losing. "I had a couple of chances," she said, "but not in the second set."

Evert says she is not obsessed with the place in history that she deserves as much for her graciousness off the court as her grittiness on it.

"When I was younger, I never thought about it," she said. "I was Just playing for that year. I know my time is limited. It sounds like I'm about to die in three years. It gets tougher mentally each year. It chips away. Sometimes I don't want to go out and be tough. I want to mellow out. I think about history more and more."

But not about the cheesecake that almost went down in history. She would not be eating any tonight. "No" she said. "Never again."