Britney Spears & Rihanna: Billboard Awards Performance!

Britney Spears joins Rihanna onstage for a performance at the 2011 Billboard Music Awards held at the MGM Grand Garden Arena on Sunday (May 22) in Las Vegas, Nev.

Rihanna, 23, started off the performance of “S&M” before Britney surprised the crowd to perform her verse from their remix version of the song.

Britney, 29, will be hitting the stage later in the show to perform “Till the World Ends” alongside her Femme Fatale tour partner Nicki Minaj.

FYI: Rihanna and Britney wore Norma Kamali “Vogue Mio” swimsuits onstage! Brit also rocked a black patent leather bunny mask custom made by Alexis Bittar.

10+ pictures inside of Britney Spears and Rihanna opening the show…