Katy Perry Brings The Hits -- And Juicy J! -- To iHeartRadio Music Festival

Perry delivers a knockout set, telling fans that an upcoming album doesn't mean 'I ain't going to play the songs you want to hear.'

Katy Perry added knee pads to her Catholic schoolgirl plaid-and-converse-sneaker ensemble for Friday's iHeartRadio Music Festival, but she probably didn't need them: The "Roar" singer delivered a knockout performance.

Opening her set with "I Kissed a Girl," Perry told fans at Las Vegas' MGM Grand (and those live-streaming from their screens) that radio "is the way that people are introduced to music — and it's free!"

Keeping with the grunge vibe, she next dove into her newest track, bringing Juicy J onstage for their duet, "Dark Horse."

With a stage covered in metallic blocks and white triangle graphics, the backdrop seemed to reflect the visuals for the cover of her October 22 album, Prism.

And the upcoming release was clearly on her mind as Perry addressed the crowd, telling them, "I have a record coming out in October, but that doesn't mean I ain't going to play the songs you want to hear."

Chart-toppers like "California Gurls," "Teenage Dream" and "Firework" followed, as she shot confetti into the audience.

The singer ended the 20-minute set with "Roar," using the same boxing ring set-up as her 2013 MTV VMA performance, jump ropes and all.

Earlier in the night — after Diddy opened the show as ringmaster — Robin Thicke performed "Blurred Lines," Elton John played classics "Tiny Dancer" and "Benny and the Jets," and J. Cole was joined onstage by TLC for their first live performance of "Crooked Smile."

Adam Lambert, Queen, and .fun closed out the first night's lineup. Saturday (September 21), the fest concludes with performances by Miley Cyrus, Justin Timberlake, Drake and more.