Sia was furious when her vocals were used on 'Titanium'.

Although the 42-year-old singer/songwriter wrote the lyrics for the track, which was produced by DJ David Guetta, she was originally angry when she discovered the vocals she recorded were used when the song was released.

Sia explained to Rolling Stone magazine: ''I had worked so hard to be this cool, credible artist. And then finally, I've retired to just work behind the scenes, and then I'm on, like, a cheesy pop house song.''

However, she changed her mind when the song went double-platinum.

Sia - who has penned songs for Rihanna, Beyonce, Britney Spears and Katy Perry - also revealed that she finds it quite easy to write for other people.

She said: ''It doesn't feel like I'm doing it, it feels like it's happening to me. It's really f**king weird!

''I used to overthink my lyrics. I really wanted to be cool. Usually, I'm writing from a character's point of view. Sometimes, I'll write one that I relate to. Those are the ones I don't give away.''

And producer Greg Kurstin says she is one of the speediest writers he has ever met.

He said: ''I've never seen anyone write a melody and lyrics that fast. She'll sing it and write it and it happens in one motion, and then she's revising. And then it's one take. You've got to keep up with her, really.''