The two-part season two finale of "The X Factor" began Wednesday night with the top three acts performing three songs each as part of their final performances.

The fierce competition between the top three acts-Rose Sonenclar, Tate Stevens and Fifth Harmony- was described by the show's co-host Khloe Kardashian as "a schoolgirl against a family man against five girls who has never given up."

Each act was given an opportunity to perform with one of their personal music idols.

Sonenclar, 13, performed a duet with country singer LeAnn Rimes. Together they sang Rimes' hit "How Do I Live." Rimes, the youngest Grammy winner ever, towered over the petite Westchester native for their collaboration.

"I love this girl so much," Rimes said afterward. "She's such an amazing talent."

Sonenclar described the opportunity as "one of the most fun moments I have had on stage in my life." Her mentor Britney Spears, who was in tears at the end of the performance, weighed as well: "They were shockingly amazing. So good!" 

Earlier in the show Sonenclar sang Nina Simone's "Feeling Good" and the judges were impressed.

"Well you are here to try and win this tonight, aren't you? I can tell," said Simon Cowell. "The first time I heard that it blew me away, none of us believed you were 13 and I still don't. Actually you sang it better tonight than you did the first time. I like your competitive spirit."

Spears said, "We are going to win this."

Cowell's contestant Fifth Harmony, a five-member girl group, was given a sixth member when Demi Lovato joined them for a performance. The former Disney Star went from judge to contestant to help Cowell's girls win the final prize. Together they sang the pop star's hit song "Give Your Heart a Break" and earned a standing ovation.

"It was so much fun," Lovato said afterward.

Cowell described it as "sensational," adding that it was what he would call "pop perfection."

Earlier in the show the girl group performed a stripped down version of The Beatles' "Let It Be." Judge L.A. Reid told them their cover of the 1960s classic was like "magic happening before our eyes." Cowell beamed with pride and called the group "five great singers," adding, "It's really important to remember that."

Tate Stevens chose "Tomorrow" by Chris Young for his solo set and one judge gave the rendition thumbs up; Cowell told him he "had a really good night." Stevens' personal music icon Garth Brooks was unable to join him onstage so he sang with country music group Little Big Town. Together they sang "Pontoon."

The second part of "The X Factor" finale will announce the winner of season two. Fans will learn who will win the $5 million prize at 8/7 p.m. Central on FOX on Dec. 20. The episode will feature guest performances by Pitbull and One Direction.

Watch Rimes perform with Sonenclar on the season two, part one, finale of "The X Factor" below.

Tags: LeAnn Rimes The X Factor USA Carly Rose Sonenclar Demi Lovato Britney Spears Simon Cowell