Fans took to social media after spotting the star speaking to Dermot O'Leary before singing in front of Simon Cowell, Cheryl Fernandez-Versini, Mel B and Louis Walsh.
Simon recognised Linzi straight away and the pair told their story of how they knew each other. Girl Thing were formed by Simon in 1999.
At the time, the group - comprised of Linzi, Anika Bostelaar, Michelle Barber, Jodi Albert and Nikki Harrington - were tipped to be the next Spice Girls but sadly their career was short lived. The band split in 2001.
After a shaky start singing a version of the Jackson 5's 'I'll Be There', Simon stopped Linzi's audition and told her to start again.
Clearly emotional and nervous, Linzi performed the song again, but she appeared to put the judges in a difficult position when it was time for them to give her feedback.
"The nerves were just killing me. I know I can sing so much better than that, it just means so much to me," said Linzi before the judges had a chance to speak.
Louis called Linzi "brave" but said the song was "a bit big" for her. However, he gave her a yes.
Former Girls Aloud star Cheryl sympathised with Linzi, saying: "It's hard because I completely feel and understand you. I feel like I've been so fortunate to have the journey I've had. I could have been in your shoes. The audition just wasn't strong, I feel it's unfair to put you through."
Linzi replied: "I do know the audition went bad. It was nerves."
Seemingly confused as to what decision to make, Mel said: "I would not normally say yes because that was not a good audition but I feel compelled to say yes to you today."
The deciding vote was down to Simon. "I'm going to be honest with you Linzi, this is not going to work out for you," he said. "Your voice isn't good enough sweetheart and I don't want to give you false hope. I know you had a hard time a few years ago and I just don't want to put you through this again."
Simon continued: "I can't lie to you Linzi. Your voice just isn't good enough and I'm going to have to say no. I'm sorry."
A tearful Linzi went over to the judges desk where she hugged Simon and Cheryl before leaving the room.
Linzi's former bandmate Michelle took to Twitter during the audition. She said: "Was not expecting that!!!! #XFactor," before later adding: "There are no words right now. Beyond shocked."
On The Big Reunion, viewers saw Girl Thing talk about their rise and sudden fall in the music industry. Their most well-known hit was 'Last One Standing', and the group performed this song during the Big Reunion concert at London's Hammersmith Apollo in February.
Girl Thing took part in the ITV2 reality show alongside Damage, 3T, Eternal, A1 and supergroup 5th Story.
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Watch Linzi's audition below: