Inside the fiercely private Grammy nominee's world

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She had one of the hottest hits of 2014 with “Chandelier” and nearly broke the Internet when the video for “Elastic Heart” dropped last week. But who is Sia, really?

The 40-year-old Aussie pop star (full name: Sia Furler) shot onto the scene in 2005 when her song “Breathe Me” was featured in the Six Feet Under finale, but she struggled with fame. The pressures of being in the public eye drove her to addiction. After contemplating suicide, the singer cleaned up and almost retired, deciding to write songs rather than perform.

“There’s a negative side of being famous. It’s not easy being in the spotlight,” says longtime friend and coproducer Greg Kurstin. “She got to a place where she wanted to look out for her best health and what’s best for her as a person. She wanted to be more in control of her life.”

She’s penned hits for Beyoncé, Rihanna and Katy Perry. But in 2011, when she co-wrote and lent vocals to David Guetta’s megahit “Titanium,” her plan to stay out of the spotlight backfired, and she just got more popular. If she was going to have a public persona, she would be in control this time.

“She doesn’t like to be famous because it infringes on her quality of life,” adds longtime manager David Russell. “She doesn’t want to sacrifice her personal life for fame. That’s ultimately not important to her.”

So when she relaunched her singing career, Sia decided to shield herself from the attention. She rarely shows her face in public and tends to use proxies who imitate her. For example, tapping Dance Moms alum Maddie Ziegler, 12, to play her mini-me in the “Chandelier” and “Elastic Heart” videos, and Girls star Lena Dunham to dance for her while she sang on Late Night with Seth Meyers.

And it’s not just a marketing ploy.

“She’s taking the issue of the image and persona head-on, and she’s redefining it,” says Tom Corson, president and COO of RCA Records, her label. “It’s very disruptive, and it’s very transformative, and she is winning doing that because the core of it is just great music and great artistry.”

Sia also caused a stir last week with the video for “Elastic Heart,” which featured Ziegler and actor Shia LaBeouf dancing half-naked in a giant cage. Some criticized the video, saying it bordered on pedophilia – remarks the video’s co-director Daniel Askill calls “ridiculous.”

“Nothing could have been further from our minds in terms of the intent of the project,” Askill says. “It’s really one of those things that is very much open to interpretation. Unfortunately, some people’s minds are a bit darker than others.”

Adds child star Ziegler, who says Sia has become “like family”: “We had so much fun on set. Shia was always laughing and cracking jokes. It was all positives.”

All the Internet controversy and unusual performance art aside, the singer is just doing this to protect her personal life.

“She’s just completely normal, you know?” says Kurstin. “We’ll talk about whatever bad reality TV show that we’re watching, food, the things that we like.”

Adds Russell: “I’ve never seen Sia happier.”

Aside from her skyrocketing career (she’s up for four Grammys this year), the artist just wed her longtime love, filmmaker Erik Anders Lang, this summer.

“That’s a big part of why she’s doing really well now,” Kurstin says. “She’s so happy in her marriage. Her husband’s great. We’re all just happy for her.”

For much more on Sia, pick up the new issue of PEOPLE, on newsstands now