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OTRC: Britney Spears and Rihanna kick off the 2011 Billboard Music Awards - What did you think?

Britney Spears and Rihanna perform at the 2011 Billboard Music Awards in Las Vegas on Sunday, May 22, 2011.

Rihanna's show-opening performance at the 2011 Billboard Music Awards of her single "S&M" was kicked up to the next level when Britney Spears hit the stage to join in on Sunday.

On the stage in Las Vegas, Rihanna was clad in a white vinyl corset while Britney donned a black one. The performance ended with a pillow fight.

The two singers previously collaborated on a remix of "S&M" back in April. Rihanna told New York radio station Z100, which premiered the new version of the song that "in order to do a remix, it had to be major."

"I asked my friends last week who they really wanted me to collaborate with and Britney was definitely one of the most popular names," Rihanna said. "It was really amazing that she really wanted to be a part of this song. It made it really really special."

The single "S&M" debuted in January and is featured on Rihanna's fifth studio album, "Loud," which was released in November 2010.

Beginning in June, Spears, 29, is set to perform concerts across the United States and Canada to promote her new album, "Femme Fatale," her seventh major record, which was released in late March and which hit No. 1 on the Billboard 200 Albums Chart in April. Check out her tour dates here.

Rihanna is set to go on a North American tour with "Forget You" singer Cee Lo Green, check out their tour dates.

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