John Lennon was "terrified" and "physically sick" after losing a bet with Elton John to join him onstage during his 28 November, 1974 show at New York's Madison Square Garden.
The Rocket Man star worked with Lennon on his Walls and Bridges album, with Elton providing backing vocals and playing the piano on its lead single Whatever Gets You Thru the Night.
Although the track wasn't a favourite of The Beatles star, Elton, along with others in the studio, convinced Lennon to release it as a lead single. The pair made a wager that if the tracked topped the Billboard Hot 100, Lennon would join Elton on stage during a performance.
The single became a massive hit and Lennon lived up to his word but, according to Elton, who opened up with Sean Ono Lennon on the recording for new BBC Radio 2 and BBC Sounds series John Lennon at 80, the Imagine hitmaker battled crippling nerves ahead of the show.
"I don't think he’d played since the Peace Concert in Toronto, to be honest with you, and certainly not in New York I don't think since The Beatles, and he was terrified," Elton told Sean.
"I can tell you that he was physically sick before the show, absolutely physically sick," he went on.
Explaining that Lennon eventually recovered from his nerves and took to the stage, Elton shared: "When he came on stage he was fine because the reception that he got at Thanksgiving Madison Square Garden in 1974… I've never heard a noise, a roar like it and… we all get goosebumps thinking about it."
He recollected: "A lot of us cried and there were tears running down our faces because here was one of the four people that were the biggest band ever."
Elton added that Lennon "wasn't nervous during the show (and) he just played the same brilliantly," once he hit the stage.