The 31-year-old judge wowed the audience by wearing a sexy, form-fitting black Herve Leger dress. With her midriff partially exposed, the tanned "Scream and Shout" singer wore her long blonde hair in loose waves. Spears completed her look with Erickson Beamon earrings and Yves Saint Laurent pumps.
During the Pepsi PreShow Live, the betrothed beauty shared new details about her 8-week-old puppy, Hannah. "She's 1.8 pounds. She is a barker! She is feisty and she will let you know when she's upset," Spears said. "And she always wants mommy!"
Spears often dresses her pooch in adorable outfits. "I love this store called Fifi & Romeo," she explained. "It's ridiculously expensive but it's fun."
Later that night, Spears' two remaining acts -- Carly Rose Sonenclar and Diamond White, both 13 -- performed two songs each. Sonenclar, who's been in the No. 1 spot for two consecutive weeks, performed Justin Bieber's "As Long As You Love Me."
"You sang it better than Justin did," judge Demi Lovato, 20, told Sonenclar. When she performed Beyonce's "If I Were a Boy,' Spears told the young musician that she is "the definition of The X Factor."
White sang covers of James Brown's "It's a Man's Man's Man's World" and Rihanna's "Diamonds." "So proud of Diamond and Carly tonight," Spears tweeted. "Wow, wow, wow."
To find out which four contestants will move on to next week's semi-finals, tune in for The X Factor Thursday, Dec. 6, at 8 p.m. EST on FOX.