Billboard Music Awards Preview: Britney Spears to Perform With Rihanna, Nicki Minaj; Beyonce to Receive Millennium Award

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Britney Spears

Rihanna leads the finalists, followed by Eminem, Lady Gaga, Bruno Mars and Justin Bieber.

It's almost showtime for the Billboard Music Awards, which are set to get under way at 8 p.m. ET on Sunday night in Las Vegas.

Rihanna leads the finalists with mentions in 18 award categories, including top Hot 100 artist, top female artist, top R&B artist and top dance/electronic artist. "Love the Way You Lie," her chart-topping collaboration with Eminem, places her as a contender in six song categories.

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Eminem is a finalist in 16 award categories, including top Billboard 200 artist, top artist, top male artist and top rap artist.

Lady Gaga, Bruno Mars and Justin Bieber also are finalists in multiple categories.

Saturday afternoon marked the first Britney Spears sighting. Entering the MGM Grand Arena wearing sweatpants, Uggs, a T-shirt and her hair up in a ponytail, she was surrounded by security but seemed to be in great spirits.

"She looked great and really fit," said an observer.

Spears will perform her latest single, " 'Til the World Ends," after which she'll be joined by tourmate Nicki Minaj for "Superbass." She also is expected to perform with Rihanna during the show.

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Of course, the pop diva of the night is Beyonce, who's being honored with the Millennium Award for her career achievements and influence in the industry. She'll be introduced by her mother, Tina, in what's said to be an emotional tribute.

Ever the perfectionist, Beyonce pushed Sunday's rehearsal and full run-through back by 90 minutes as she ran through her acceptance five times. 

Meanwhile, Cee Lo Green is taking on one of the more technically ambitious performances of the night. Without giving too much away, it involves a weighty musical instrument defying gravity.

The 2011 Billboard Music Awards airs from 8-11 p.m. ET/PT on ABC.