Gretta Ray only rehearsed Mumford & Sons' 'Forever' hours before joining the band on stage to perform their song.
The 21-year-old singer/songwriter supported Marcus Mumford and co on the Australian leg of their 'Delta World Tour' in January, and has recalled the "surreal experience" of getting to duet with them on the track from their recent album 'Delta' and how she only had a short space of time backstage to prepare for the first live performance in Sydney.
Asked whose idea it was to perform 'Forever', Gretta exclusively told BANG Showbiz: "I think Marcus was the one who suggested it be that song.
"'Forever' is a new song of theirs, one which was not initially part of their setlist for the tour.
"The first time we sung it together was at the Enmore Theatre show in Sydney, and they had planned on changing the setlist a little in order to include that particular song anyway."
Gretta - who performs at All Points East on the same bill as the 'I Will Wait' hitmakers at London's Victoria Park on June 1 - admitted she was very "nervous" but the 'Little Lion Man' group were good at making sure the performance felt "natural" by ensuring the spontaneous moment was chilled and "fun".
She said: "It was a pretty quick turn around; I had to learn the song backstage and in soundcheck that day before the gig.
"So I was a little nervous! But I felt that the band just wanted me to get out there and have fun singing the song, not stress about it too much.
"As a result, the performance ended up feeling really natural. We had a great time."
Gretta had "imagined" making a surprise appearance during their headline set and admitted she never thought she would actually get to perform with the Grammy-winners.
She said: "The idea of joining the band on stage for a song was something that I had imagined, but not something I was certain would actually happen.
"However, they seemed equally as excited about the possibility of us singing together, so it was a mutual decision in the end."
Gretta plays a headline show with a full live band at London's Electrowerkz on Tuesday (16.04.19).