What happens when rock royalty goes on vacation and winds up center stage at a major festival? Lisbon’s Rock in Rio found out Thursday night when the Rolling Stones welcomed a surprise guest during their headlining set: none other than Bruce Springsteen. The Boss stepped out with guitar in hand to perform a fiery duet with Mick Jagger on the Stones’ classic “Tumbling Dice,” electrifying the Portuguese crowd with a rendition full of swagger and soul.

Hints of the collaboration surfaced earlier in the day when Patti Scialfa, Springsteen’s wife, shared a nostalgic Instagram post tagging Lisbon and featuring a #ThrowbackThursday photo of herself, Bruce, and Jagger—sparking speculation among fans.

This marked only the second live appearance by the Stones since pausing their Australian tour in the wake of L’Wren Scott’s death, a loss that deeply affected frontman Mick Jagger. Their previous concert, held in Oslo, Norway, was notable too—E Street Band guitarist Steven Van Zandt was in the audience, on break from filming his Netflix show Lilyhammer. As the Stones continue their “14 on Fire” tour with a stop in Zurich on June 1st, fans may want to keep an eye out—perhaps Nils Lofgren could be the next surprise guest.

Notably, this isn’t the first time Springsteen and the Stones have shared a stage. In December 2012, during the Stones’ Newark, New Jersey performance at the Prudential Center, Bruce joined them for the same song, “Tumbling Dice,” during a concert packed with guest stars like Lady Gaga, the Black Keys, and ex-Stones guitarist Mick Taylor. That show was broadcast live on pay-per-view.

Even outside of collaborations, Springsteen has long shown his appreciation for the Stones’ legacy. Over the years, E Street Band setlists have included covers of “The Last Time,” “Satisfaction,” “Jumpin’ Jack Flash,” and a near-nightly run of “Street Fighting Man” during their 1984–85 tour. In 2010, Bruce also teamed up with Alejandro Escovedo at New Jersey’s Stone Pony to perform “Beast of Burden.”