The 41-year-old diva took to Twitter where she posted a picture of the "Jealous" hitmaker playing a guitar.
"Working with the extraordinary @labrinth is like a relaxing meal at a hipster cafe," tweeted Sia.
"The Greatest" singer, who was standing opposite a mirror, made sure her face was not visible while clicking his snap.
Neither Sia nor Labrinth have revealed the details of their project together.
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