Eminem -- Performs 'Stan' With Rihanna at Lollapalooza

Rihanna and her apparent BFF Eminem performed some of their hit songs together at Lollapalooza Friday night ... but the show stealing moment was their rendition of "Stan."

It all went down during Em's headlining gig at the end of the first day of the annual festival ... when Rihanna hit the stage looking smoking hot as usual.

Rihanna replaced Dido's famous vocals on the hit song -- and while she might not hit the notes like Dido … Dido never had an ass like RiRi's.