Britney Spears to Sex Up Billboard Music Awards

Britney Spears

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Who better to raise the curtain on the 2011 Billboard Music Awards ? which return Sunday night after a five-year hiatus ? than Britney Spears, who earlier in the afternoon was seen at a dress rehearsal with Rihanna, sources close to the show tell PEOPLE.

"She looked fabulous," one of the sources says about Spears, "better than ever. The opening is sexy and playful, Britney's's got a fantastic entrance. It's epic."

Live on the stage of the MGM Grand Garden Arena Las Vegas, Spears will perform "Til the World Ends" with nominee Nicki Minaj after the latter delivers her hit "Superbass" backed by a slew of hot male dancers.

The show will air live on ABC, with Hangover 2 star Ken Jeong as host. Previously announced among the performers are Cee Lo Green, Pitbull, Jennifer Lopez, the Black Eyed Peas and Keith Urban.

Nominees include:

Top Artist: Justin Bieber, Lady Gaga, Rihanna, and Taylor Swift.

Top New Artist: Justin Bieber, Taio Cruz, Ke$ha, Bruno Mars, and Nicki Minaj.

Top Hot 100 Song: Taio Cruz, "Dynamite"; Eminem featuring Rihanna, "Love the Way You Lie"; Bruno Mars, "Just the Way You Are"; Katy Perry featuring Snoop Dogg, "California Gurls"; and Usher featuring Will.I.Am, "OMG".

Top 200 Album: Justin Bieber, My World 2.0; Susan Boyle, The Gift; Eminem, Recovery; Lady Antebellum, Need You Now; and Taylor Swift, Speak Now.

Top Country Artist: Jason Aldean, Kenny Chesney, Lady Antebellum, Taylor Swift, and the Zac Brown Band.

Top Rock Artist: Kings of Leon, Linkin Park, Mumford and Sons, Muse, and Train.

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