Jack Johnson honoured Mumford and Sons as he stepped in for the British band at the Bonnaroo music festival in Tennessee.
The Hawaiian singer-songwriter was a last-minute headlining addition to the four-day event after Mumford and Sons had to cancel their appearance on June 15, due to bassist Ted Dwane's brain surgery.
"We want to dedicate this whole set to Ted Dwane and all of Mumford and Sons," Jack added, as the thousands-strong crowd roared their approval back.
He added: "I wrote this song this morning, and I just wanted to share it with you guys."
The song, titled Bonnaroo, was a recap of the situation involving Ted undergoing surgery for a blood clot on the surface of his brain, forcing the folk-rock stars to cancel their Saturday night performance at the festival. They also cancelled a number of US shows to allow Ted to recover.
Jack - who performed at the first-ever Bonnaroo 12 years ago and also headlined the festival in 2008 - admitted he was nervous about taking to the stage at the festival.
"As soon as I said yes, I said, 'Holy s**t. I gotta learn a lot of chords and a lot of words really quickly," he said.
Meanwhile, Ted has shown off his freshly shaved head after being discharged from hospital. The group are due to headline Glastonbury at the end of the month.
"Bear with a sore head! Thanks so much for all the well wishing, it seems to be working! I'm home," he wrote on the band's official website.
Tom Petty And The Heartbreakers closed the Bonnaroo festival on June 16, with Sir Paul McCartney, Bjork, R Kelly and Wu-Tang Clan also performing at the festival.