Billie Jean Won Wimbledon. Randy Struck Out Dodgers. A Baseball Sibling Rivalry for the Ages. – The Wall Street Journal

“Yeah, chicken wings,” said Randy, 72. 

“There we go,” Billie Jean said, laughing.  

Surely you know about the “Battle of the Sexes”—and the legendary career of Billie Jean King, who won dozens of major tennis championships, became one of the most consequential athletes of all time, and not long ago, with her partner, Ilana Kloss, took a minority ownership stake in baseball’s Los Angeles Dodgers. 

But Billie Jean wasn’t the only Moffitt who played in a professional sporting event on Sept. 20, 1973. 

Earlier in the afternoon, right-hander Randy Moffitt took to the hill at Candlestick Park for the San Francisco Giants, throwing a perfect third of an inning of middle relief as the Giants prevailed over the “Big Red Machine” Cincinnati Reds.

“A day game,” Randy recalled crisply. 

“Randy, you didn’t tell me that!” Billie Jean shot back. It’s a small detail missing from her wonderful, recently-published autobiography, entitled “All In.” 

“I could have put that in my book! Oh, nuts.” 

It was an early moment in Randy’s impressively sturdy baseball career. He’d wind up playing 10 of his 12 Major League seasons with San Francisco, four times finishing in the National League’s top 10 in saves. 

While his sister ranks among tennis’s greatest, Randy’s Giants contribution is commemorated at Oracle Park with a plaque on the club’s “Wall of Fame” alongside icons like Willie Mays, Willie McCovey and Juan Marichal. 

“On King Street,” Billie Jean said, chuckling. 

The Moffitt siblings jumped on the phone with me to talk about the Best-of-5 Divisional Series between the Dodgers and the Giants, which is currently tied at one game apiece. 

While the brother-sister warmth was clearly there, one of baseball’s most heated regional rivalries couldn’t be denied. 

Billie Jean bleeds Dodger Blue. 

Randy’s a Giant for life. 

“Very much pulling for the Giants,” Randy said. “It was a thing that went all the way back to A baseball—the dislike for the Dodgers.”

Still, the Moffitts grew up Dodger fans on West 36th St. in Long Beach, Calif. Their father, Bill, was a talented basketball player, so basketball was their first sport, but they soon fell in love with baseball. Billie Jean remembers her father reading box scores and going to see the Pacific Coast League Angels host the Hollywood Stars in nearby Wrigley Field. 

“The Hollywood team wore Bermudas,” Billie Jean said. “Pinstriped Bermudas. Remember that, Randy?”

Randy recalled seeing the new-from-Brooklyn Dodgers play at the L.A. Memorial Coliseum. 

“We sat in the knothole section, all the way up,” Randy said.

They were ballplayers both—Billie Jean played baseball and softball, batting leadoff, starting at shortstop and later taking up the racket sport she’d eventually dominate and transform. Randy would grow to 6 foot 3, becoming a fireballing high school ace the Giants drafted in 1970. 

That year, Randy was playing instructional league when a visiting Willie Mays asked him if he needed anything for the apartment he was sharing with fellow minor leaguers Gary Matthews, Garry Maddox and Horace Speed. 

“I said, ‘Well, we could use a television set,’” and [Willie] said, ‘Your glove looks worn,’ ” Randy recalled. “Next morning, I had a new glove and a TV sitting in my locker.”

“Willie did that for me.”

As the Moffitts ascended their respective sports, they kept a close watch on each other’s accomplishments. Billie Jean, five years older, got a head start—by the time Randy hit the majors, she’d already won many major championships. They’d follow each other’s results in newspapers, and talk on the telephone when they could. 

A specialist reliever ahead of his time, Randy made only one start in his career—at Dodger Stadium, it turns out. In June 1974 he scattered 11 hits and struck out seven Dodgers over seven innings for the Giants in a game L.A. would win 4-3 in 10. 

“Had a lot of men on base, but I seemed to work out of it pretty well,” he recalled.

Billie Jean, meanwhile, was preparing for Wimbledon, where she’d win a mixed doubles trophy with Owen Davidson. 

“Obviously, we’re Dodger kids,” Billie Jean said. “But when Randy was playing, Randy came first. That was it. Done. I was for the Giants when Randy was with them.” 

Even as a Dodgers owner, Billie Jean gives a loving nod to her brother’s legacy. King wears a Dodgers jersey with the No. 17—Randy’s number with the Giants, which he picked to commemorate the number of clubs that passed over him in the draft. 

“I was upset with the Dodgers because they didn’t draft Randy,” Billie Jean admitted. “Ticked off, to be honest.” 

In “All In,” Billie Jean writes that while she was getting ready to face Bobby Riggs in the Astrodome, she took comfort in the fact that she was getting dressed in the visitor’s clubhouse, the same space Randy and his Giants teammates used.

“I felt connected to Randy,” she told me. “It was very grounding.”

In the lead-up to his sister’s match, Randy hung a sign over his home locker at Candlestick: I WILL TAKE ALL BETS. 

His Giants teammates loaded up with bets on Riggs to win, “just to get under my skin.”

The next day, a proud Randy Moffitt returned to the ballpark to collect.

“You guys owe me a lot of money,” he said, laughing. 

Write to Jason Gay at Jason.Gay@wsj.com