The 12 finalists - the girls, the boys, the groups and the over-25s - all took to the stage on Saturday night to perform 80s classics.
One of the highlight performances of the night was prison guard Sam Bailey's rendition of 'The Power Of Love', which left Nicole Scherzinger "speechless".
She added: "I don't even know what I just witnessed and what I just watched.
"I'm like "ah shhhhh....shenanigans". If this is what you are starting out with where do we go from here?"
Gary Barlow told her: "That was totally off the clock. You are in a complete league of your own - what a performance."
Louis Walsh said she "hit every single note", while proud mentor Sharon said she had "goosebumps all over".
But the show ended on a tense note with the first flash vote of the series.
Shelley Smith was told she was in the bottom two for Sunday night's sing off after receiving the lowest number of votes from the public. Voting lines were opened for only 10 minutes before the result was announced at the end of the show.
We have got to find a way to try and modernise your voice.
The bubbly singer performed a rendition of the Heart classic Alone, but Gary felt that despite the theme of the week, she still sounded a little bit "dated" and wanted the singer to "modernise" her voice.
He added: "What everybody is doing here tonight is a modern twist on the 80s and I felt you were stuck in the 80s."
But fellow judge Louis told her: "You are a great, great belter and you really took off with that performance - it was fantastic."
He added: "It was cheesy but it was good cheese."
Nicole said she had "made a total transformation".
Following the result the 34-year-old told host Dermot O'Leary she was going to "sing her little heart out" during Sunday's sing off.
"I have got so much more to give," she added.
Earlier student Tamera Foster, 16, received great comments following her performance of Chaka Khan's 'Ain't Nobody'.
Sharon told the young singer she was a very "exotic" act, while mentor Nicole described her as a "force to be reckoned with".
Gary told her: "You have the X Factor", while Louis called her an "out of the box pop star".
Meanwhile, Gary and Nicole were critical of Sam Callahan, 19, who sang Summer Of 69, while Nicholas McDonald, 16, sang True by Spandau Ballet
Band Kingsland Road burst into a performance of Wham!'s I'm Your Man which was packed with choreography.
Supermarket worker Abi Alton, 19, sang Livin' On A Prayer while playing a white piano, while Lorna Simpson, 26, from London, sang So Emotional.
There were mixed comments for Band Miss Dynamix and Luke Friend, 17, who sang I'll be Watching You.
Sharon told the boy: "Your voice isn't the greatest but you don't need it with that face."
A-level student Hannah Barrett opened the show with a rendition of What's Love Got To Do With It, while Rough Copy closed it with In The Air Tonight.
Who will join Shelley in the sing off? Find out during the results show on Sunday on UTV from 8pm.