Paul McCartney brought out a surprise guest during his set at the Desert Trip festival this past weekend: Rihanna, the pop star with whom he collaborated on the 2015 song "FourFiveSeconds."

The single version also featured Kanye West, who didn't join his collaborators onstage last night. You can watch a fan-shot video of McCartney and Rihanna's performance of their song from Desert Trip above.

“We have finally found somebody under the age of 50!” McCartney told the audience. And as Billboard notes, 70-year-old Neil Young was the youngest artist to play Desert Trip, the four-night, two-weekend classic-rock festival that also included sets by Bob Dylan, the Rolling Stones, Roger Waters and the Who.

"FourFiveSeconds" showed up about midway through McCartney's shows, in between a pair of Beatles classics, "Lady Madonna" and "Eleanor Rigby." His set was filled with songs by his old band. Likewise, all of the performers at Desert Trip pretty much kept to their best-known songs during the much-hyped festival.

Even Dylan, who won a Nobel Prize last week in between his two Desert Trip appearances, played a guitar onstage for the first time in four years, as he kept his set lists close to his classic '60s material.

“It’s like a once in a lifetime thing," McCartney told the audience during his show last night. "Or for us, twice in a lifetime,” he added, referring to the two-weekend festival, which included repeat performances by all artists both weekends.

Desert Trip’s Best Photos