Jaden Smith duets live with Bieber
The 14-year-old, the son of actors Will and Jada Pinkett Smith, sang Fairytale with the Canadian pop star in Washington.
"Great show tonight !!! Thanks DC! Proud of my bros @CodySimpson and @officialjaden tonight. They killed it. Me and jaden even played #fairytale," Justin tweeted.
He later re-posted a picture of the pair on stage.
He will take to the stage alongside mentor Usher, Beauty And The Beat collaborator Nicki Minaj and Taylor Swift at the glitzy ceremony in Los Angeles' Nokia Theatre on November 18. Other performers include Christina Aguilera, Pink, Kesha and Pitbull.
Rihanna and Nicki lead the nods with four nominations each, while Justin trails behind the two hip-hop princesses with three - tying him with British boyband One Direction.