Early in her quarterfinal match at Wimbledon today, Billie Jean King, who is going on 40, found herself acting, oh, 14. Bobbing up and down as Kathy Jordan dawdled before each serve. Trying to anticipate. A fighter anxious to get on with the fight.

"Then I told myself: 'You'll be exhausted by the third set,' " she said later.

And when she walked off Court 1 a winner after two sets, King was loads less emotional than the occasion seemed to merit. When others are ready to hand her a shawl and knitting needles, King burns for a 21st Wimbledon title. If not in singles, then perhaps in mixed doubles.

"I'll always dream about getting to the finals," she said. "Even when I'm 80."

One imagines the Wimbledon wires humming late in the third decade of the 21st century . . . "Billie Jean King today defeated the granddaughter of Chris Evert . . . "

This will be her 14th time in the Wimbledon singles semis. Dwell on that a bit. Several of the prominent teeny-boppers of tennis aren't much older than that. She has retired one more time than Tracy Austin has made the final here.

"I was worried about quitting on top," King said of a decision made after winning her sixth women's singles title here in 1975. So much did she miss the sport she overhauled that King came here the next year just to practice. And get set to unretire.

Her attitude now is Sam Snead's before his eyes finally went bad. Retire? Snead always huffed. What's a fella do when he retires? He plays golf and fishes. "Or what I'm doin' now," he'd add.

Once King played tennis to live. It's the other way around now. And this Wimbledon is just wacky enough so that an hour or so before King's grand victory, her classy contemporary, Virginia Wade, lost a chance to fossilize the other half of the semis.

"Two steps short on everything," said Wade, who will be 38 in less than another fortnight. "Not really alert. I don't think I ever lost to her (Yvonne Vermaak of South Africa) before.

"A day off would have helped. My (round of 32) match ended about 6:45 last night (and her quarterfinal one began promptly at 2 this afternoon). If there had been a little more energy, I'd have been able to cope."

Now England's major Wimbledon hope is gone again. Our Ginny, as the papers call her, one story going so far as to label her "the grand old lady of tennis."

What's that make King? Old enough to be asked, legitimately, about her contribution to posterity, to all those cute little clones who hit two-fisted backhands from their cribs near the baseline. Posterity? King feigned incredulity.

"I haven't started yet," she shot back. She's had more knee surgery than many a veteran NFL linebacker: each decade or so brings a new crusade to lead. Still, the will to work at playing a game rarely flickers.

"When we lost the (women's) doubles," she said, "Chris said that might help me in singles. I said maybe she thought that way, but I didn't. To me, doubles here is just as important as singles."

With age has come perspective. "When you're young," she said, "you're the whole universe; when you're old, you realize you're just a speck. But there's still a lotta joy. I was injured so much from '76 through '81. But I've been healthy lately, enjoying it more."

"You have to be in really good shape here, be able to lunge for volleys and stay low. Every match I've gotten a little bit better. (In response to another indelicate question) obviously, I've declined. I'm no longer No. 1."

In truth, she was rated 25th coming into Wimbledon. "But I'm one of the luckiest people in the world to have played tennis. Would Nureyev stop dancing? Would Sinatra stop singing? I like tennis."

One of the whims for King is to come back in another tennis life and realize all of what she made possible. "You don't have to lose anything as a human being," she said.

"But the kids today have all the vehicles I didn't. Coaches. Videotapes. I wouldn't have had such a lousy forehand." She made an ugly face, then smiled. "I should have been born in '73," she said, "instead of '43."

Wade talked of having experienced a fantastic last six months, "with quite a bit of tennis without tournaments. Doing what I like without the pressure. I'm pretty happy about my Wimbledon . . . Let's be serious."

King got very serious about that form of self-motivation. Lower expectatons do lift considerable weight from a player's shoulders, she admitted, adding that she always wanted to tote as much as possible.

"I'd rather make a goal and fall flat trying to get it," she said, "than not to make it at all. I'd have lost a long time ago with that attitude."

This is the 10th anniversary of that sociological phenomenon known as King-Riggs, when her victory in the Houston Astrodome over Bobby Riggs answered the cosmic question of whether a relatively young woman pro could beat an old tennis hustler.

"It wasn't that big a deal to beat Riggs," she said. King vowed at the time never to participate in a rematch: she promises that promise will be kept. Promoters are trying to coax her into breaking it.

"Bobby's been talking to me for years," she said.