The classical singer, who has previously battled with nerves, sang a rendition of Nella Fatasia and earned a standing ovation from the four BGT judges.
"That was the most beautiful, haunting piece of music I have heard," Amanda Holden told her. "I think there is a huge gap in the market for this and I think you are the girl to fill it."
Simon Cowell agreed, commenting: "That was incredible, you look like an absolute star and I am so happy we found you on Britain's Got Talent."
"It was faultless," added David Walliams.
Lucy will be joined in the final by French trio Yanis Marshall, Arnaud and Mehdi, who closed Wednesday's semi-final by performing another slick dance routine in killer high heels.
"You made that look so easy!" Alesha Dixon told them.
Simon said: "That was the cherry on top of the nuttiest show we have ever done on Britain's Got Talent."
Amanda was also impressed with the group describing their routines as "sexy" and "sassy" before complimenting the men on their "best legs ever".
Sisters that was fierce!
David Walliams to Yanis Marshall, Arnaud and Mehdi
The dancers earned their spot in the BGT final after being pitted against fellow dance group Cartel, who had a surprise in store for their performance.
During their initial audition Simon told the five-piece group that member Jamie did not fit in with the act.
And in their pre-performance video, Cartel appeared as a four-piece, telling the audience Jamie had decided to leave the group as a result of Simon's comments.
But this was soon revealed to be untrue as Jamie joined the boys on stage following a spectacular entrance and even pretended to be unable to get up off the stage at the end of the routine as a nod to Simon's comment that he was too old to be in the group.
"It has made me so happy that you have proved Simon wrong," said David Walliams.
"That was the happiest moment and you deserve your place in the group," added Amanda.
But Alesha and Simon both agreed that Jamie would be better suited as Cartel's choreographer as opposed to a dancer, with Alesha telling him: "You were lagging during the performance, it just wasn't on that higher level."
Simon told him: "I do think I was right and I am just doing my job.
You are like the Rocky (Balboa) of the dance world.
Simon Cowell to Cartel's Jamie
"You would be better suited behind the scenes as opposed to the spotlight."
But during the judges' vote for the second spot in the final Simon backed Cartel, along with Amanda, while David and Alesha voted for Yanis Marshall, Arnaud and Mehdi.
The decision was brought back to the public and the French dance trio won their place.
Alesha's golden buzzer act REAformed earned high praise for their performance on Wednesday night, but failed to progress further in the competition.
The girl group sand a rendition of Beyoncé's End Of Time, with Amanda telling them: "Your performance was faultless, the harmonies were perfect, it was amazing."
"I am so proud of you," added Alesha. "And it was so brave of you to tackle a Beyoncé song."
Also performing during Wednesday's semi-final was singer Ellis Chick, burlesque group Crazy Rouge, mother and daughter act Kitty& Rosie, artist Brian Chan and ventriloquist Sam Jones.
Britain's Got Talent returns to UTV on Thursday from 7.30pm.