Eminem and Rihanna gave fans a taste of what to expect on their co-headlining Monster tour when they took the stage together at Lollapalooza on Friday night.

Rihanna joined the rapper on stage at his headlining set in Chicago's Grant Park to perform their hit duets Love The Way You Lie (2010) and The Monster (2013). They also surprised the crowd with a rendition of Eminem's 1999 hit, Stan. Rihanna sang the chorus originally performed by British singer Dido.

"Thank you @Eminem and Chicago for letting me share this moment with you at #Lollapalooza," Rihanna tweeted after the show.

Eminem -- who also headlined the annual musical festival in 2011 -- also brought out his Bad Meets Evil partner, rapper Royce da 5'9".

Rihanna and Eminem have been busy preparing for the upcoming mini-tour. The 41-year-old posted a photo with the We Found Love singer on July 29 with the caption, "The Monster rehearsals."

Aside from experiencing great music, fans will also be able to leave each show with some special goods.

"I am thrilled to announce that #TRAPSTAR teamed up with us on the OFFICIAL MERCHANDISE exclusive to #THEMONSTERTOUR Can't wait til u see dis," Rihanna wrote on Twitter.

The Monster tour will kick off on Thursday, Aug. 7 at the Rose Bowl in Pasadena, Calif. and will continue until Saturday, Aug. 23 at Comerica Park in Detroit, Mich. See the full dates here.

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Eminem, Rihanna