King still volleys with abandon, but what comes after tennis?

Even as she continues to seek that elusive last big triumph on the court, Billie Jean King is looking ahead toward life after tennis. Last year she came as close to the former goal as she has in a long time, staging a dramatic series of upsets to reach the semifinals at Wimbledon before losing to Chris Evert Lloyd. For many at King's career stage that might have been a twilight accomplishment worth savoring. But Billie Jean is not the type to be satisfied with anything less than victory, no matter what the circumstances.

''It wasn't good enough though, was it?'' she said when an interviewer recalled that moment. ''I intend to play again this year - and if I do, I want to win it.''

And beyond that?

''After July, I'll reevaluate what I want to do with the second half of my life,'' she said during an appearance in Boston where she is playing in this week's women's tour event. ''I know I want it to be something involving motivation. I think people tend to sell themselves short and have low self-esteem - especially women. I like to see people do better than they think they can do, and I think I can help.''

Billie Jean, of course, has never had that problem. On the contrary, she's been an eternal optimist regarding both her game and her longtime role as a standard-bearer for equal rights. And if she hasn't always won, she's scored enough victories on both fronts to assure her a permanent place in the annals of tennis and of the women's movement.

From her debut at Wimbledon as 18-year-old Billie Jean Moffitt in 1961 (she lost her first match), through her incredible record of 20 titles there including six singles championships, to that gallant bid for one more crown last summer at age 38, King has regularly been on center stage at this most prestigious of all tennis tournaments.

And there have been many other triumphs - four US Open titles; scores of victories all over the globe including the French, Italian, and Australian Opens; four times ranked No. 1 in the world; a record 18 times in the US Top Ten; first female athlete to earn more than $100,000 in a season (1971); etc., etc. And although it was mainly for hype and money, who can forget her victory over Bobby Riggs in their famed ''Battle of the Sexes''?

Early on, King began battling for women's rights too, or, perhaps more accurately, for equal opportunity for all.

''I don't think of myself as 'for women,' '' she said. ''I think of myself as for people - men and women. That's what women's liberation is all about: for all people to get the opportunity to do the best they can.''

But historically women were frequently denied such opportunities - so that's where her battleground lay. In 1968 she and three others formed the first women's pro troupe. Next she helped establish the first major women's tour. Then she led a long and ultimately successful battle to gain equal prize money for women at the US Open. No wonder that in the '70s it was generally agreed that the two most recognizable American athletes were Muhammad Ali and Billie Jean King, or that today's top female players - even those who may be at odds with her - readily give her the lion's share of credit for today's big prize money and endorsement opportunities.

Of course all this seemed in the past when she ''retired'' in 1975, and especially when she did so again in '81. But each time her need to compete was too great to give up just yet.

''I still had the ability,'' she says now. ''And I missed the game. As you get older, you acquire a different perspective. You realize the joy and love of what you do. It's like a Horowitz playing the piano, or a Nureyev dancing. You keep remembering how much you like it.''

She never had, after all, depended on things like size or strength - in both of which she was far below many of her rivals. She always did have a pretty fair amount of overall athletic ability, but clearly her biggest asset was her tremendous will to win - and that still rages within her as powerfully as ever.

That was apparent in last spring's Italian Open, where she saved an apparently lost first round match and went on to reach the semifinals. And at Wimbledon she followed basically the same script, saving three match points against Tanya Harford, then upsetting Wendy Turnbull and Tracy Austin before losing to Evert Lloyd. And although it's been eight years now since she last won a major tournament (Wimbledon 1975), the fire still seems to be there as she plays various selected events while looking forward to one more try for a big one this summer.

She is also now playing in the Women's Tennis Classic, a new tour for players over 30, and in the current scaled-down version of Team Tennis run by her husband, Larry.

''I believe in the concept,'' King said of the latter venture, which failed in a more big-time approach and is now in its third year as a smaller, less costly operation. ''It helps men and women work together, and teaches the importance of things like cooperation and being supportive. I think it would be good culturally over the long run. But if it makes it, it makes it - and if it doesn't, it doesn't. I believe in free enterprise.''

Asked to compare her own longevity in the game with the recent abdication of Bjorn Borg, who was a toddler when she was already playing big-time tennis, King replied:

''You have to take each person on his own terms. When the women's pro tour started I was 27. That gives you perspective. Also, since 1970 the prize money has grown so enormously. Borg has made millions. When I was his age, I was broke.

''That's not why I stayed in it. I love the game. But I just want to point out that Borg has had a very different experience than I've had. I think it bothers him that he's not No. 1 anymore. That has happened to others. If his experience had been like mine, I think he'd be fighting tooth and nail to be back up there. But maybe he'll be back. Who knows?''

King, of course, knows all about ''retiring'' and coming back - but even as she eyes her busy 1983 schedule, see seems resigned that the next time will probably be for good. And although she doesn't indicate anything more definite than that vague hope of ''motivating'' people, she seems ready for her next challenge, whatever it turns out to be.