The "Roar" singer was spiralling down after the break-up, which coincided with the failure of her last album, "Witness", and the "Chandelier" singer really understood what she was going through as a great depression kicked in.
Sia, who has been open about her own mental health struggles, tells the Los Angeles Times Katy turned up at her house and she was in "a bad way."
"She had a real breakdown," the Australian star explains. "She's on stage with 10 candied lollipops and clowns and dancers, selling the dream, the joy, the happiness - and that's really hard sometimes when you're not feeling it yourself."
"I knew she was driven and ambitious, that was clear from the beginning. But I didn't realize that she was so reliant on that validation for her psychological well-being. She did say 'I feel lost.' I think it was a big kick to her ego, but it was the best thing that could have ever happened to her, really, because now she can make music for the fun of it. Getting number ones does nothing for your inside."
Perry recently revealed recording her new album, "Smile", and a reunion with Bloom, who is now her fiance, helped pull her out of a career and life low.
Sia admits she was a big part of the couple finding love again, revealing, "I'd be on the phone with Orlando and have call waiting with Katy trying to call me."
Katy is now expecting the actor's daughter, who is due in days. This will be Perry's first child, while Bloom has a nine-year-old son, called Flynn, from his marriage to model Miranda Kerr.