What do you get when you pair the most famous teenager in the world with a sultry vocal powerhouse? You get one smokin' episode of 'X Factor' in the U.K.!

Justin Bieber performed his romantic Christmas carol, 'Under the Mistletoe,' for the British audience. Clad in a jacket that looked part Michael Jackson, part Nutcracker, the Biebs kept it simple, from the lighting to the choreography (or lack thereof).

Backed by a live band and a snowy Christmas backdrop, the Canadian simply crooned into the mic -- and occasionally spun the stand around -- for the bulk of his performance. Later, he took a seat on the stairs onstage and sang, then strolled over to the judges' table to serenade Kelly Rowland and Tulisa Contostavlos. Selena Gomez, get your boy back in line!

Meanwhile, judge and mentor Kelly Rowland pulled out all the bells and whistles (and cleavage!) she could for her onstage stint. "I've seen a lot and I've heard a lot, and I wasn't sure what to expect. But now I can feel the love. And I want to give it back. I love you, U.K.," Rowland said in a brief introductory voice over. "Ready? Here. I. Am."

Backed by dancers, lasers and heavy beats, Rowland then began singing her hit with David Guetta, 'When Love Takes Over,' before breaking into 'Down for Whatever,' followed by an outro dance to 'Commander.' While Rowland's dancing was impeccable, her vocals sounded flat -- though in her defense, belting is difficult when you're shaking and shuffling your stuff all over a stage. Rowland's fellow judges were impressed with her performance. "You put it down, girl!" Louis Barlow said. "You remind me of a young Tina Turner."

Watch Justin Bieber Perform on the U.K.'s 'X Factor'

Watch Kelly Rowland Perform on the U.K.'s 'X Factor'