Emily Middlemas has been voted off the X Factor, leaving Matt Terry, Saara Aalto and 5 After Midnight to battle it out in next week's finale.
The 18-year-old was given the boot after losing out in a surprising sing-off with Matt.
After learning her fate, she said: "I'm so proud of myself to actually get to the semi-final, this year has been amazing and I'm just so lucky to be part of it."
The sing-off, described as a "shocker" by host Dermot O'Leary because neither acts have been in the bottom two before, saw Matt perform Hurt by Christina Aguilera and Emily appear barefoot to sing Wings by Birdy.
Dermot revealed this week had seen the biggest vote of the series so far, with more than two million votes cast.
Judges Sharon Osbourne, Louis Walsh and Matt's mentor Nicole Scherzinger all voted to save Matt, while only Emily's mentor Simon Cowell wanted to keep her in the show.
Simon said: "The three do deserve to be there, but I'm gutted for Emily to get this close. It's all about what you do with your life now and using this properly."
The show opened with all the semi-finalists performing Band Aid's Do They Know It's Christmas?, before Sweden's Got Talent winner Zara Larsson performed her singles I Would Like and Lush Life.
Later in the show, Lady Gaga performed her new single Million Reasons on a platform, clutching a cowboy hat and dressed in a black jacket with crystal embellishments which she threw off to reveal a plunging chainmail top and leather shorts.
After the performance, Dermot asked her about her English accent, which he had heard during the soundcheck, and she put on the voice, saying: "It's a really awful one where I talk like this."
Throughout the show, the names of the finalists were daubed on battle buses the hopefuls will travel round the country in over the next week as they bid to drum up votes.
All the finalists will also perform homecoming gigs, with Terry appearing in Bromley on Wednesday, boyband 5 After Midnight will hope to draw a crowd in Leicester on Tuesday, and Aalto will return to her native Finland for a gig in Helsinki on Monday.
The show was the last to be filmed at London's Fountain Studios, the home of the competition for 13 years. The live final will take place at the nearby Wembley Arena.
Simon paid tribute to studio staff, saying: "They are going to knock this place down, I can't believe it, but that is life I guess."
Earlier in the day Dermot posted a picture of the studio on Instagram with the caption: "The last ever live day at Fountain Studios today.
"It's been our home for 13 years. And it's where I taped my first pilot show. Just a tiny piece of concrete in north-west London. But memories are history, laundry and property. Thanks to all the crew and staff for making it spit and crackle."
Sharon's act Aalto has taken over from Matt as the bookies' favourite to win after her performances of Mariah Carey's All I Want For Christmas Is You and Sia's hit Chandelier on Saturday.