Louisa has consistently impressed the judges with her performances and last weekend reduced her mentor Rita Ora to tears.
Following her rendition of Everybody's Free from Romeo & Juliet, Ora said: "I'm so happy that this came to life."
Cowell also heaped praise on Louisa, saying: "This really means something to you, doesn't it? You're not just in it to come second or third, you really want to win this.
"This is a performance you're going to be remembered for, I'm telling you."