Billie Jean King, winner of more matches (203) and more titles (20) than any other player in Wimbledon's 103-year history, spent a good deal of time at the All England Lawn Tennis and Croquet Club last week, preparing for her 20th appearance in the oldest and most prestigious of tennis tournaments.

She roamed the club's verdant and picturesque 12-acre grounds, practiced on the green and spongy grass, and sat in the empty seats of the Centre Court -- "just soaking up the atmosphere," she said, "and trying to get the vibes."

Wimbledon is the tournament King dreamed about as a teen-ager in California. Now she is an important part of its history -- winner of six titles in singles, 10 in doubles, four in mixed doubles. She feels an extraordinary affection for and kinship with the All England Club, where tradition, as British writer Rex Bellamy once put it exquisitely, "hangs like a sunlit mist over those green lawns in the land where tennis was born."

BJK is 36 now, battle-scared but hardly battle-weary.

She is working like a teen-ager again, practicing as much as she can, riding a stationary bicycle 50 minutes a day and lifting weights in her suite at the Gloucester Hotel, where the management kindly stores all her training paraphernalia from year to year so that she can set up a private mini-gym before and during Wimbledon.

King is seeded No. 5 in singles, but has an exceedingly tough draw. In order to win the title, she probably would have to beat Sylvia Hanika or Pam Shriver in the fourth round, defending champion Martina Navratilova in the quarterfinals, Chris Evert Lloyd in the semis and Tracy Austin or Evonne Goolagong Cawley in the final.

Few people give her a chance, and she can't blame them. She knows she has much better prospects for adding to her proud record collection in doubles (she and Navratilova are favored to defend their title) and in mixed doubles (BJK and Dick Stockton are seeded No. 2).

Still, Billie Jean King is remarkably resilient, ever a wily and ferocious competitor, and so she dreams about another singles trophy.

"It really depends on how I play. I'm really short on match play, but I feel if I can get through my first match (she has a first-round bye, and then will play Texan Anne Smith) and remember what I have to do on a fast court, I could be quite dangerous.

"I'm quite happy now," she said, briefly adopting a practiced British accent, "and playing quite nicely, thank you.

"I've also promised myself to keep my mouth shut," she went on. "I may have a hard time keeping that promise, but I'm going to try. If I do, I'm telling you, I could do really well. I just don't want to get upset over anything that happend on the court. If I get a bad call or something, I'm going to forget about it and just keep going. I've really been thinking a lot about that.

"Sometimes older players will throw up their hands when it gets tough.Maybe it's windy and rainy and they get a bad call, and they just go, 'I've been through this too many times. What do I need this for?'

"I think when you start feeling that way, it's time to stop playing. That's a good indication you should just bag it, or else just go out there and play for fun -- throw the ball up and roll your arm over and don't get unglued if you lose. But I'm not there yet. I still want to go out on court fully prepared to win physically and mentally."

Long a crusader for various causes, King has become a champion of the sports world's over-35 gang. She took heart at 40-year-old Jack Nicklaus' recent triumph in the U.S. Open golf championship, which she watched with unbridled glee on television in her hotel room.

"I know most people think that at 36, you haven't got a chance. They've given up. They gave up on Nicklaus a few times, too," she said.

"God, he hit the greatest putt on the 17th hole that last day," she continued, savoring again the 20-footer that effectively clinched Nicklaus' fourth Open title.

"I was running around the room, limping up an down, because he had kind of faltered the day before and I was thinking: 'Geez, you can do it. Don't think you can't just because everybody's been telling you 15,000 times a day that you're over the hill.'

"Then, oh, he dug so deep on that one putt. He was standing over the hall, and just by looking at him you could tell he was going to make it. I said, 'Look at him, he's going to do it, he knows this is the shot that's going to win it or lose it.' That's what makes a champion. He knew this was the moment he had to come through, and he did.

"It was a fantastic moment. I got goosebumps. Every day I think I should send him a letter or telegram, and then I get embarrassed. But he was an inspiration for me. A lot of people say you can't do anything after you're 30 or 35, and that kind of negative thinking can get to you if you let it."

King has been pained by this attitude, but resolutely has refused to let it get to her since she embarked on her latest comeback, following surgery on her left foot on Dec. 22, 1978.

It was this delicate, 3 1/2-hour operation -- to remove a vestigal bone and bone spur and repair a damaged plantar fascia (the dense, fibrous membrane of the sole that binds together the deeper structures of the foot) -- that most people assumed would end her glorious career.

It has not been easy since Detroit and Houston. There she reacquanted herself with the euphoria of winning. She realized that the long hours of sweat and pain in the gym and on the practice court -- the preparation for which she knows there are no shortcuts -- were paying off. She was still among the best half-dozen women players in the world, and on a given day she could beat the best.

Since then, ailments and illness have impeded her progress. Her results have been spotty.

This spring she was plagued by bronchial asthma and had to pull out of four tournaments. Inactivity undermined the form she had so carefully cultivated. She wanted to do well in her first French Open in eight years, but never found her rhythm on the slow clay in Paris and was beaten in the quarterfinals earlier this month by Australian Dianne Fromholtz.

From Paris, she returned to the United States for exhaustive tests at the Cleveland Clinic.

"I had been feeling awful in Paris. I was sleeping 16 hours a day, I didn't feel like getting out of bed to practice, so I figured that was ridiculous, I'd better get a complete checkup," she said. "They fixed me up and organized the medication for my asthma, so now I feel great. It's a big relief anyway, knowing there's nothing seriously wrong with me."

And now she is back at Wimbledon, where she first played in 1961. She was Billie Jean Moffitt then, a chubby chatterbox who teamed with Karen Hantze to win the women's doubles at 17, the youngest doubles champion in Wimbledon history.

She has not missed a Wimbledon since, despite three interruptions of her career by knee operations. She did not play singles in 1976, having "retired" after winning her sixth singles title the previous year, but has been in the doubles and mixed doubles annually.

"Some of the teen-age kids now hear that this is my 20th Wimbledon, and they can't believe it. They think I should be in the museum or something.

"The kids keep teasing me that I'll be here when I'm 50, but I know my days are numbered," she said.

"There will be a time to quit. It's something you have to recognize for yourself, though. No one else can tell you. I know it's coming. But I'd like to keep playing as long as I enjoy it, and can play up to my own standards."