At the heart of any athletic act is self-conception, the chiseling of your body and character. In that sense, Ruth Bader Ginsburg had the soul of an athlete, not much different from the great female champion of her generation, Billie Jean King. They started their work at the same time in the early 1970s and moved toward the same end from opposite directions, one with her arm and the other with her head: King stropping volleys at the net in blue suede sneakers and sequined wide lapels, all dressed-up muscle and discontent; Ginsburg with that deceptive, slim-shouldered stillness in her blue suits, such a steeliness of the brain that she could “take a lawyer who is making a ridiculous argument and just shake him like a dog with a bone,” her friend Antonin Scalia said.

This week, the late Supreme Court justice becomes the first woman to lie in state at the Capitol, and King will celebrate the 50-year anniversary of the women’s tennis tour she risked her career to spearhead in 1970 for just a $1 contract. The two women only knew each other late in life, but they became friends. On Friday evening, King was on a Zoom call with her fellow eight founders of the Women’s Tennis Association, reminiscing about what provoked it all, when she learned of Ginsburg’s death. The line fell absolutely silent, the pall visible.

What was it about this little slip of a woman that made so many all-time great athletes, from Megan Rapinoe to Tom Brady, recognize her? No other Supreme Court justice has received such an outpouring from athletes. And it’s not just because she was a workout fiend, though it was rousing to see an 87-year-old justice doing planks and throwing around a 12-pound medicine ball. Like “a Mighty Mouse,” as King says, laughing.

It’s because Ginsburg understood that in so many interactions, lack of muscle or power is misinterpreted as a defect, a weakness across the board, mental and emotional flimsiness. Ginsburg fought that implicit deficit. There was strength in her words, strength in her visage, strength in her arguments — just as there was in King, who whipped Bobby Riggs 47 years ago this week at a time when, as King once put it with some hyperbole, America believed women were “chokers and spastics who couldn’t take pressure. Except, of course, in childbirth.”

The train of Ginsburg’s legal thought was similarly outright brawny. She once wrote that “Sex like race is a visible, immutable characteristic bearing no necessary relationship to ability.” Now there is a statement that rings the athletic bell.

“I hung on her words,” King says.

The more you look at Ginsburg’s body of work — and the way she trained her body to keep working — the more you realize that “feminist” was much too small of a word for her. Yes, she arrived in the early 1970s, just in time to rescue the rest of us from all of the things that plagued girls back then, the fear of obstacles, and of ostracism, that resulted in so much cautious, furtive channeling. “We were on a tightrope,” as King says. “I had to be careful how I explained something so as not to lose the audience.” At Cornell as an undergrad, Ginsburg had to “suppress” how smart she was, she once said.

What Ginsburg did, and King, too, was show you what performance under pressure looked like. Theirs were no abstract arguments, rife with futile, disputatious pedantry over whether to call themselves Mrs. or Ms. — feminism “from the neck up,” as King calls it. There was no suggestion, either, that you had to treat wifedom and motherhood as a false destiny. There was just rubber meeting road.

When King won her first Wimbledon title, all she got was a clothing gift certificate. By 1970, she was furious that Ilie Nastase got $3,500 for winning the Italian Open while she got just $600, and she was sick of being at the mercy of male tournament promoters who made her play before noon in empty stadiums. So she risked Grand Slam suspension to start a women’s tour, and by 1973 she was the first female athlete to make $100,000. But she still couldn’t get a credit card without the signature of the husband she was supporting.

That’s where Ginsburg stepped in. The same year, Ginsburg won her first landmark pay discrimination case before the Supreme Court, Frontiero v. Richardson, on behalf of a female Air Force lieutenant who was denied a housing allowance for the husband she out-earned. Before making her oral argument, Ginsburg didn’t “dare” eat lunch because of the stomach-churning pressure: If she buckled or made an error, it would have been a failure “for all women,” she said. It was the Black Quarterback Syndrome.

But with a voice as calm and steady as the turning of a page, she destroyed rationalizations for sexism, step by step, illogic by illogic. She would go on to win five of the six cases she argued before the Supreme Court — most by overwhelming margins.

“People who are great winners love process,” King says. “They set the clear goal, and then it’s, ‘How do you get there?’ One ball at a time. That’s how you win a match. That’s what she did.”

Ginsburg was, unmistakably, a competitor. You could see that in her taste for unlikely friendships with conservative virtuosos such as Theodore Olson and Scalia, whose dissents, she once said, made her better because he “nailed all the weak spots.” You could see it, too, in the way she worked out in her last years, while beating back cancer and heart surgery. She exercised so hard, she told the New Republic in 2014, because it sharpened her. She bragged that she was “number one” on the court in the average speed with which she wrote her opinions.

What made Ginsburg such a heavyweight is that hers was not some feeble orthodoxy that couldn’t survive a tough challenger. Even her pro-choiceness, whether you agreed with it or not, was really just a stance against the institutional weakening of a woman: “When government controls that decision for her, she’s being treated as less than a fully human adult responsible for her own choices,” she explained. It’s a critical point to understand, maybe the most meaningful one: Ginsburg wanted women to have choices — but she never told anyone what choice to make. She just wanted “you to be your authentic self,” King says.

That freedom to self-fashion is the through line of Ginsburg’s life and career. From the start, when she was a Brooklyn tomboy nicknamed Kicky who leaped from garage roofs with the neighborhood guys. To the legal scholar, who wrote the momentous 1996 decision opening the Virginia Military Institute to women, discarding “generalizations about ‘the way women are’ ” in favor of “talent and capacity.” To the bantam octogenarian, who loved parasailing and hit the weight room the day after she broke three ribs in a fall.

When her friend Nina Totenberg of NPR learned Ginsburg could do 20 pushups, she said: “Those aren’t real pushups, right? Those are girl pushups,” Totenberg recounted in the documentary “RBG.”

Ginsburg replied, “Oh, they’re real.”

Yes, they were.