The Australian singer - who has recently found success collaborating with David Guetta on the track 'Titanium' - wrote the track 'Blank Page' for Christina, and is delighted at the way it sounds.
She tweeted: ''So @TheRealXtina Sounds amazing on our next collaboration. Blank page. Squeeeeee (sic)''
It is not the first time Christina has worked with Sia - she also wrote the track 'You Lost Me' on Christina's 'Bionic' album.
Speaking about her forthcoming seventh studio album recently, Christina confirmed it will be a ''rebirth'' for her.
In a series of posts on twitter, she wrote: ''Album title: Lotus. Representing an unbreakable flower that survives under the hardest conditions and still thrives.
''Self expression and freedom [are album themes]. The album represents a rebirth for me. (sic)''
She also explained there would be many of her trademark ballads on the record.
Christina added: ''There will be a 'Lotus' intro to set the tone for the whole record. Conceptually I didn't target one particular sound/genre.
''There are some beautiful personal ballads on this album. I wouldn't do a record without them.''