Lollapalooza Day 1 ended with a major dilemma for fans: Festival organizers C3 Presents decided to pit Eminem, Arctic Monkeys, Zedd and Phantogram against each other for the night’s closing set. Those who chose Eminem, one of the biggest names on the bill, got a huge treat when Rihanna rose from the stage for a surprise performance. The crowd lost it.

Rihanna joined Eminem for three songs and the duo teased their upcoming “Monster Tour.” Together, they played their collaborative hits: “Love The Way You Lie” and “The Monster.” In yet another surprise, Rihanna stepped in to sing female vocals on “Stan,” Eminem’s 2000 duet with Dido, which was the major highlight of her performance (and his whole set).

It was Eminem’s first time back at Lollapalooza since 2011, and he played fan favorites like “Without Me,” “My Name Is,” “Kill You” and “The Real Slim Shady” as expected. He ended his performance with “Not Afraid,” accompanied by a guitar solo, and “Lose Yourself.”

Eminem and Rihanna’s “The Monster” tour kicks off at the Rose Bowl in Los Angeles on Aug. 7. See fan-shot footage of the Lollapalooza performance below.