The British singer was the support act on the 'Firework' hitmaker's US leg of her 'California Dreams' tour and admits she was in awe of the performance Katy puts on every night.
She said: "I watched Katy's show and was just like, 'Oh my goodness.' I was just blown away by everything."
Ellie - who also performed at the Duke and Duchess of Cambridge's wedding reception in April - was also apprehensive that Katy's fans wouldn't cheer for her when she got on stage, and was pleasantly surprised by the warm reception she got.
She told MTV: "It's really nice to hear my songs in a stadium when they perhaps wouldn't be at this point. It's not often that I hear my songs so epically big and loud. I can pretend to be a rock star for a night!"
"I genuinely had nightmares of just finishing a song and it being like (awkward clap). But actually they were really up for it!"
"Any time I moved or rocked out, they'd all cheer," she recalled. "It was a really warm reaction. When I went out and was wandering around, loads of people came up and wanted pictures and stuff."