Britney Spears failed to impress with her lacklustre comeback performance at the MTV Video Music Awards on Sunday night.The singer opened the glitzy ceremony at Las Vegas, Pearl Theatre at the Palms Casino Resort by lip-synching her new song 'Gimme More'.Wearing a sparkly black bra and hot pants, the 25-year-old mother-of-two looked bleary eyed and unfit as she struggled to keep up with her energetic backing dancers, who threw themselves around stripper poles in an S+M-inspired set.Cameras revealed celebrity guests, including 50 Cent, scoffing at Britney's performance, while Rihanna was seen in fits of laughter.Following the song, comedienne Sarah Silverman came on stage and sarcastically said: "Was that incredible? Britney Spears, everyone. Wow. She is amazing. She is 25-years-old and she's already accomplished everything she's going to accomplish in her life. It's mind blowing." MTV officials had reportedly warned Britney, who famously shared a same-sex kiss with Madonna at the 2003 VMAs - to keep her routine family friendly, or risk being kicked off the show. Rihanna and Britney's ex-boyfriend Justin Timberlake were the night's big winners.Rihanna took home two of the most coveted awards, Video of the Year and Monster Single of the Year for her smash hit 'Umbrella', while Justin was named Male Artist of the year and Quadruple Threat of the Year. During his acceptance speech for Male Artist of the Year, the heartthrob pop star took the opportunity to criticise MTV for replacing music with reality TV shows. He said: "I want to challenge MTV right now to play more videos!"Other winners included Black Eyed Peas singer Fergie, who scooped Female Artist of the Year, Beyoncé Knowles and Shakira who won Most Earth-Shattering Collaboration for their song 'Beautiful Liar', Gym Class Heroes who were named Best New Artist and Fall Out Boy who walked away with Best Group.