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.

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