The track, which was written by Sia herself, originally featured the vocals of Mary J Blige, but that version was scrapped in favour of the 'Clap Your Hands' star's.
'Titanium' is now available on iTunes as part of the countdown to Guetta's fifth studio album Nothing but the Beat, which is released on August 29.
Sia wrote on Twitter: "It's amazing that 40 minutes of my life and zero promo turned into my most successful song to date. I should write more songs intended for others."
'Titanium' currently sits at number five on the iTunes chart in Sia's native Australia, number nine in the UK and number 28 in the US.
Sia has previously written with Christina Aguilera and recently revealed plans to work with Beyoncé in the future.
Listen to David Guetta featuring Sia - 'Titanium' below: