Joan Jett Loves the New York Liberty. The Feeling Is Mutual.

Joan Jett’s unmistakable voice was carrying, and she was pretty sure it was working some magic.

The New York Liberty had taken a slim lead against the Las Vegas Aces in the third quarter of Game 3 of the W.N.B.A. finals on Sunday, and the Rock & Roll Hall of Famer was doing her part, bellowing along with the crowd’s “De-fense” chant from her courtside perch at Brooklyn’s Barclays Center. When the Aces started to go cold, Jett took it as a sign.

“I’m hoping they recognize my voice and I’m messing up their shot,” the husky-throated musician said, using an expletive. “It’s all mental, you know what I’m saying?”

It was a must-win contest for the Liberty, who were down 2-0 in the best-of-five series. As Jett kept up her boisterous chant, the Aces missed six consecutive shots. The Liberty went on an 8-0 run, and the diminutive singer and guitarist jumped up to high-five the 6-foot-3 former Liberty center Sue Wicks, a friend.

Some 10 years had passed since Jett last attended a W.N.B.A. game (her summer touring schedule got in the way), but she fell quickly back into the playoff delirium she had enjoyed as a courtside fixture in the late 1990s and early ’00s, when the team made the final round of the playoffs four times but failed to win a title.

The rock star said she first fell for the game in 1996 when the N.C.A.A. asked her permission to use Joan Jett and the Blackhearts’ cover of “Love Is All Around” to promote the women’s basketball tournament. The following year, the W.N.B.A. began its first season and Jett bought Liberty season tickets, often showing up to big games with a red cloth voodoo doll she used to taunt opposing players.

“She’d hold it up and stab that dang thing!” Teresa Weatherspoon, the former Liberty guard, said during halftime. “When you talk about the Liberty, you have to mention Joan’s name. Any battle we had on the floor, Joan was in it with us.”

Jett grew up a self-described tomboy in Rockville, Md., and became a fan of Major League Baseball’s Baltimore Orioles at age 11, after her father took her to see the pitcher Jim Palmer throw a no-hitter. Her intersection with sports continues today: She still follows the Orioles faithfully, and is known to set up livestreams on the drum riser during shows so she can follow along. The theme song for “Sunday Night Football,” is an adapted version of the Blackhearts hit “I Hate Myself for Loving You,” performed by Carrie Underwood.

During her early days of W.N.B.A. fandom, Jett opted to sit directly behind the bench instead of courtside with the other celebrities. (“It just feels more inside basketball to me,” Jett said. “You can hear the coaches talking.”) The Liberty would slap her hand on their way onto the floor. Jett occasionally came to practices, and once even flew to Houston with the team for a finals game.

Jett developed particularly close friendships with Weatherspoon and Wicks, who remembers being so star-struck the first time she saw Jett at Madison Square Garden, where the Liberty initially played, that she almost knocked over Rebecca Lobo, the team’s center. Wicks had a copy of “The Hit List,” Jett’s 1990 album, while playing overseas in Europe, and said it had been a “great friend” to her during lonely stretches abroad. “For me, she’s a goddess,” Wicks said.

In 1999, Ray Castoldi, the Garden’s organist, asked Jett and the Blackhearts to record “Unfinished Business,” a song he had written for the Liberty after their crushing finals loss that year. Jett not only cut the track the following season, but filmed a video with the team and performed the song at halftime during a game.

“It’s hard to explain the energy,” Jett said of those early years. “I was on the outside looking in, but they made me feel like I was on the inside. It was a fun, really inclusive time.”

Jett feels a natural kinship with athletes, who, like longtime touring bands, travel with a tight-knit team and are expected to perform on command. And like the athletes in the W.N.B.A., who have carved out a professional place for themselves while expanding the public’s idea of what women are capable of doing, Jett broke down boundaries in music: battling to prove to record labels and crowds that she deserved to be a frontwoman despite her prodigious talent. “We’re people that could relate to what each other was doing,” she said.

Crystal Robinson, a former Liberty forward with whom Jett remains close, said the recognition was mutual: “For us, it was just the fact that she supported us,” she said. “She was fighting that female battle before we started. We had this camaraderie.”

Jett’s return to the Liberty on Sunday was an overdue homecoming. Before the game, she nursed a beer as she held court with Wicks and Robinson at a table in the Barclays’ V.I.P. lounge. The recently retired W.N.B.A. star Sue Bird came by to pay her respects, as did the actors Jason Sudeikis and Michael Shannon, who portrayed Kim Fowley, the manager of Jett’s band, the Runaways, in a 2010 film.

As the restaurant emptied before game time, Jett got restless. “I feel like we’re missing stuff!” she said giddily, before heading toward the court to find her seat. Just before tipoff, Becky Hammon, the Aces head coach who had been a Liberty guard in her playing days, spotted Jett taking a photo of her from across the court and struck a quick pose.

Once the game started, Jett was up out of her seat to cheer on nearly every Liberty point. She gleefully taunted Hammon after a Jonquel Jones bucket (“Three-pointer, Becky!”), and debated foul calls with Wicks and Robinson. When Jones blocked a shot from the Aces star A’ja Wilson in the third quarter, Jett removed her black jean jacket to cheers from the crowd. “It’s hot in here!” she shouted back.

After the Aces went cold in the third quarter, the Liberty stretched their lead. “I feel good,” Jett said. “But they’ve broken my heart before.”

She appeared on the Jumbotron soon after, gamely swinging a Liberty towel overhead as “I Love Rock ’n Roll” blared on the public address system. Then, she fired T-shirts into the crowd with an air cannon, with the crowd roaring for her.

“I felt the love,” Jett said. But she was mainly focused on her potential as a tactical influence: “It reminds Las Vegas that I’m here, and that can make them nervous.”

She needn’t have worried. The Liberty found their rhythm in the second half and defeated the Aces, 87-73, extending the series to a Game 4, which will be played in Brooklyn on Wednesday. Should the team force a Game 5, it will play for the franchise’s elusive, first-ever title.

“You’ve got to be back Wednesday!” a fan told Jett as the clock wound down. “You’re clearly the good luck charm.”

But Jett is prepared for any outcome. “That’s the nature of being a sports fan,” she said. “To be there through the tough times and the good times.”