Carole King will be joined by the cast of Beautiful - The Carole King Musical for the final song of her set at her Barclaycard British Summer Time gig in Hyde Park this evening.
Cassidy Janson, who plays Carole in the show at London's Aldwych Theatre, will perform a duet of I Feel The Earth Move with the 74-year-old in front of 50,000 fans.
Cassidy, who has played the singer-songwriter in the story of her life since November, said: "Usually my heart goes into my mouth just before I go on stage but mostly I am just excited.
"Who gets to sing in Hyde Park? I am dueting with a living legend. It's such a big thing I am only just taking it on board."
The pair met for the first time at a secret rehearsal on Friday.
Cassidy said: "It was amazing and I cried. She couldn't have been more warm and encouraging. She held my hands and gave me a hug it was quite overwhelming. She was so gracious.
"She's really something - she's 74 and so full of energy."
The pair rehearsed together before Carole invited Cassidy to watch her rehearse.
Cassidy said: "Singing together it was pretty surreal. When I was singing you could feel her watching and wanting me to enjoy it. I couldn't stop looking at her and you could see how much she loves the music.
"I feel so honoured to get to play someone who has this huge talent and huge heart and her songs still sound current years later. That's a hell of a legacy."
Carole will be performing her album Tapestry in its entirety for the first time ever. It includes the first song she sold at the age of 17 - Will You Love Me Tomorrow, as well as Natural Woman, Beautiful and You've Got A Friend.