It was past tea time on Thursday, and the breeze blowing in across the English Channel brought a chilly nightcap to what had been the sunniest, most pleasant day of the week in the old resort town of Eastbourne, 65 miles south of London.

Chris Evert, Rosemary Casals and U.S. Federation Cup captain Vicki Berner buttoned their coats as they piled into the car that would take them from the tennis courts to the Cavendish, one of the cavernous, ornate old hotels that line the gray water front in this vacation place.

Meanwhile, Billie Jean King, 33 and feeling frisky as a teenager on her surgically repaired right knee, put her U.S. team blazer on top of her white track suit and headed back toward the grass courts of Devonshire Park. "Are you coming with us?" yelled Berner to King.

"No, I'm staying," replied King. "I only played 12 games today. I want to practice."

BJK had done her grueling daily exercise routine and practiced in the morning, outclassed Frederiqe Thibault, 6-0 6-0, after lunch, and then sat at courtside giving tidbits of captainly advice as Evert and Casals completed a straight-set romp over France.

On Friday, King overwhelmed Greer Stevens, the improving 20-year-old South African, 6-2, 6-0. Saturday it was a 6-1, 2-6, 6-2, triumph over Diane Fromholtz, helping the Americans win the Federation Cup over Australia.

King knows how to harness and channel the driving energy that churns inside the 5-foot-4, 135-pound body that loves to feel obsessed. She is, in her own self-analysis, very goal-oriented and very highly motivated.

Right now, her goal is her seventh Wimbledon singles title. And she is a hyper as she ever was in the days before her temporary retirement from singles in July, 1975.

"I just want to be a great athlete again," she says. "I love being a jock. It's right for me. It's the thing that gets me and my talents in sync.

"The only thing I haven't quite figured out is how you relax when you have a sense of purpose. When I get a sense of purpose, I get so tense I can hardly breathe."

Obsession. It is apparent in every conversation with King, who almost single-handedly dragged and talked women's tennis into prominence by sheer force of personality, whose bright blue eyes still sparkle with wonder even though she has been through enough crusades for anyone's lifetime.

It is evident in her energy and enthusiasm, which seem to know no human bounds, and it is unmistakeable when she steps onto a tennis court, where her spirit soars as both athlete and artist, competitor and unrelenting technical perfectionist.

As that chilly early evening breeze drove others off the court on Thursday, King went out and found a willing player, Ingrid Bentzer, who was already out on court No. 5, practicing serves.

"We ought to get some other balls. These are heavy," said Brenner.

"They're round aren't they? These are fine," said BJK as she camped in the forecourt to warm up with some brisk volleying. She drilled a forehand as she talked.

BJK always chatters during practice. She talks to the person across the net, to bystanders, to herself. It is an animated monologue, complete with sound effects.

"You've got to love it" . . . thwack . . . "With all your heart" thwack . . . "with all your soul" . . . thwack . . . "with all your might." Thwack. She netted a volley while backing off slightly from a bullet rifled straight at her and threw her racket into the net in self-deprecation. "I'm doing the same thing, I'm ducking," she said. Then she laughed, and added in a wonderful falsetto British accent, 'Shocking, simply shocking."

She spotted Bentzer lagging in the back court, territory that should be vacated at the first opportunity on grass courts with their fast, low, skiddy and often capricious bounces and teased her with a drop shot. "If I had your Swedish team, I'd run your little buns off," King squealed gleefully.

As with practically everything else in her life, practice for King is a giddy whirl of motion and emotion. She doesn't do anything unless she can do it with a passion. Her third knee operation last November made her free, mobile and healthy enough to end her fitful semiretirement. And so she has thrown herself full tilt back into the sport that has been her obsession since at the age of 11 she stepped off a public court in Long Beach, Calif., after her first lesson and told her mother I'm going to be the best player in the world.

For King, tennis does not mean merely winning matches. Satisfaction also comes from hitting the ball well, playing points correctly, immersing herself totally in the game and its history. Total involvement is one of her favorite themes.