The 31-year-old was eliminated from the competition after viewers voted her into the bottom two along with Kitty Brucknell. Both acts are mentored by veteran judge Louis Walsh.
In the sing-off, Brookes performed '(You Make Me Feel Like) A Natural Woman' by Aretha Franklin, as Brucknell reprised her original audition song, Lady GaGa's 'The Edge Of Glory'.
Host Dermot O'Leary then threw voting over to the panel, asking Gary Barlow to make the first decision.
"This is tough," he declared. "Sami, that voice is undeniable, but I've always found you a little bit dated. Kitty, I'm a fan of yours. I'm excited by you."
He finally opted to send Brookes home, as did Kelly Rowland, who described Brucknell as a "wonderful entertainer".
Tulisa Contostavlos was next, first stating that both gave "amazing performances" in the sing-off, before explaining that Brookes was the person she "most connect[ed] with on stage" and that she would have to eliminate Brucknell.
In a surprise turn, Walsh then declined to send the vote to deadlock, instead naming Brookes as the act of his that would be sent home.
He cited Brucknell's "work ethic" when making his decision, but warned that she would have to do a better job "connecting with the public".
Speaking after the result, Brookes said: "This experience has just been amazing. I may not be able to sell a million albums as Gary said, but I am going to sing for the rest of my life."
Tulisa Contostavlos last week became the first judge to lose an act through the public vote when boyband Nu Vibe were eliminated over Frankie Cocozza.
The X Factor continues next Saturday at 8pm on ITV1.
> Johnny Robinson performed best on X Factor, say Digital Spy readers