The song is to be the lead single from the Chandelier hitmaker's seventh record, This Is Acting, which is mainly composed of tracks she wrote for other artists.
Alive is written from the perspective of the Rolling In The Deep star, and Sia admits it feels weird to be singing about someone else's life.
She tells RTV Slovejna, "The first single is entitled Alive...The song I wrote together with Adele, but it was rejected at the last minute. It's about (Adele's) life, so I now sing a song from (Adele's) perspective."
Alive is set for release on 25 September (15), while reports suggest Adele's new record, the follow-up to her smash hit 21, could go on sale in November (15).