Each finalist performed three songs - their Song of the Season, a celebrity duet, and the winner's single.
First onto the stage was Britney Spears' act Carly Rose Sonenclar, who sang Feeling Good, the song that wowed the judges at the audition phase.
She was later joined by LeAnn Rimes for a performance of How Do I Live, before impressing audience and judges alike with an "angelic" rendition of Hallelujah.
On LA Reid's team, country singer Tate Stevens' Song of the Season was Anything Goes by Randy Houser, which Demi Lovato described as a "stadium-worthy performance". He went on to duet with Little Big Town, singing Pontoon, and rounded off the evening performing Chris Young's Tomorrow.
Finally, Simon Cowell will be hoping Fifth Harmony did enough to grab the top spot. The girls kicked off with Ellie Goulding's Anything Could Happen, and were joined by judge Demi Lovato for their version of Give Your Heart A Break - a performance described as "pop perfection" by SiCo himself.
Fifth Harmony then closed the show with show-stopper Let It Be by The Beatles.
Tonight's live final will see the second series winner crowned, and will feature guest star performances by Pitbull and One Direction.
The two-hour Fox spectacular will also be LA Reid's last appearance, after it was confirmed that he would be leaving the show.