“I just want people to feel like they can do anything when they hear this,” says the six-time Grammy nominee of creating the anthem for the tennis icon and gay rights activist.

"It was a little bit of a cosmic nod," says Sara Bareilles of how she came to write her song "If I Dare" for Fox Searchlight's Battle of the Sexes. Directors Jonathan Dayton and Valerie Faris, who were in need of an original song for their finished biopic about Billie Jean King, happen to be the next-door neighbors of Jessie Nelson, Bareilles' writing partner on her Tony-nominated musical Waitress. "The universe was like, 'Hey! Take a look at this!' " Introductions were made and a screening was set.

Starring Emma Stone and Steve Carell, Battle of the Sexes focuses on King's efforts to win the same pay as her male counterparts, while simultaneously dealing with the personal journey of coming out. "I was, of course, aware of her as a feminist and a gay rights icon, but I didn't know the ins and outs of her personal story," says Bareilles, 38. She didn't know about the 1973 Battle of the Sexes tennis match ("It was a little bit before my time") between King and self-branded misogynist Bobby Riggs that acts as the climax of the film. But, she says, "I walked away thinking, 'What a badass!' "

Before setting out to write, Bareilles was able to meet King. "She is so dedicated to human rights issue, and you see that it comes from such a genuine place," says Bareilles of the tennis icon. The songwriter asked King what she, personally, wanted the song to be. "[Billie] said, 'I just want people to feel like they can do anything when they hear this,' " says Bareilles, "which is a tall order."

Composer Nicholas Britell, who was nominated for his work on 2017's best picture winner Moonlight, wrote a placeholder piece of music for the end title sequence, which Bareilles used as a jumping-off point. "But Jon and Val and Nick gave me the latitude in terms of what the song ended up sounding like," notes Bareilles, who also wrote all the lyrics to "If I Dare."

Of the actual writing process, Bareilles says the biggest difficulty was dancing the delicate line between the film's political and personal themes. The songwriter's hope is "that people are able to hear how much courage it takes for someone to ask for a little bit more than what they have." The six-time Grammy nominee sings over the film's credits: "If I dare to ask it / Then I dare it to be true."

This story first appeared in a December stand-alone issue of The Hollywood Reporter magazine. To receive the magazine, click here to subscribe.