Emma Stone found filming 'Battle of the Sexes' "super challenging".

The 28-year-old actress portrays the American professional tennis player Billie Jean King in the upcoming biography, but the star has revealed she struggled with the movie because she has "never" played the role of a "real person" in a film before.

Speaking to The Hollywood Reporter about her latest venture, Emma said: "I've never played a real person before and it's, like, this person is an icon and an absolute badass that I could never even hope to be exactly like.

"Capturing the essence was very important to us. It was super challenging."

But Emma has admitted Billie made the movie "much easier" to make because she was "generous" with her story.

She explained: "But [King] was so warm and welcoming and open to all of it. She was generous with her story that it made it much easier than if she was like, 'Please don't make this movie about me, good night.'"

The 73-year-old sportswoman is happy with Emma's portrayal of her, and thinks the 'La La Land' star has made her "look so good" in the production.

Billie said: "Emma, thanks for making me look so good."

And the co-director, Valerie Faris, felt it was "important" for the success of the movie that Emma and Billie met, although Emma felt "weird" when the two were introduced to one another.

The creative mastermind said: "It was important they met, but they were starting to get a little friendly and Emma realised, 'I think I need to have a little distance.' Emma said she felt weird because she had to just study videotape of Billie Jean. She needed to hear her voice at 29."

And fellow director Jonathan Dayton believes the "little distance" has helped Emma to "embrace" her character.

She said: "We were making a film about one year in [King's] life. It was good to have a little distance, and now we can all really embrace her."