Sharon Osbourne abstained from voting on the X Factor results show after two of her acts found themselves in the sing-off.
Student Lorna Simpson, 26, from London, became the first act to exit this year's competition.
She lost in the sing-off to Shelley Smith, a 34-year-old single mother from Devon, who is also in Osbourne's over-25s category.
After the two women sang to stay in the show, and it was time for the judges to say who they would like to send home, Osbourne said to host Dermot O'Leary: "Dermot, I abstain."
Louis Walsh said it was an "amazing" sing-off, and said the person he would like to send home was Simpson.
Smith knew she was going to be in the sing-off since last night after she lost a "flash vote" that resulted in her receiving the fewest phone votes out of all the acts.
Gary Barlow said it was a "fantastic" sing-off, and told Smith that if she had sang last night as well as she sang tonight she would not have been in the nail-biting scenario.
Barlow opted to send Smith home, but Nicole Scherzinger said she wanted to send Simpson home and so her fate was sealed.
Simpson said her favourite memory of the competition was when she got through to the boot camp stage.
Reflecting on the result, her mentor Osbourne said: "For once in my life I'm stumped."
Pop stars Ellie Goulding and Cher performed on the show.
Last night, Barlow confirmed he is to step down from the talent contest at the end of this series, making the announcement mid-way through the live show.
The Take That frontman, mentoring the groups, commended three-piece Rough Copy as they closed the evening with a cover of the Phil Collins classic In The Air Tonight.
"I'm so glad for my last year on the show, I get to mentor you," he said. His confirmation follows speculation that this would be his final series as a judge on the show.
Elaborating on his departure in a statement later, Barlow, who is to embark on a solo tour next year, said: "I've had an amazing time on X Factor these past three years.
"A show like this needs to stay fresh and exciting so it's a great opportunity for someone to come in and take the show past its incredible 10-year anniversary.
"Next year is looking to be a very busy year for me and (The X Factor) is a full-time job so it felt like the right time to hand over the baton.
"I've loved being a part of the show and have met and worked with some fantastic people but most importantly, more than ever, I want to go out with the winning act this year. Come on the groups!"