Jessie had previously been criticised by her fellow coaches for even putting Ruth through at the auditions.
Will complained at the time: "[Ruth] was all over the place."
While Danny added: “She was singing in a different key to the song.”
Speaking ahead of the performance against Ben, Ruth Ann admitted: "I feel lucky to even be here right now, for this battle I've got to prove myself, more than anyone else, I'm going to totally bring it."
The duo performed Whitney Houston hit I Wanna Dance With Somebody together, before Jessie delivered her shock choice.
She said: "The person I’m going to be taking through to the live shows is Ruth Ann. I believe in you."
Her fellow coaches weren't impressed.
"That was one of the biggest shocks and upset I've seen," Danny told Will.
But Jessie hit back: "Oh shut up you guys."
Watch [tag]Ruth Ann St Luce[/tag] battle on The Voice UK with Ben Kelly below...