Justin Bieber -- BALLED OUT AT KANYE'S BDAY ... Says NBA Champ

Justin Bieber is NO JOKE on the basketball court ... and absolutely dominated the A-list competition at Kanye West's birthday party ... so says an NBA witness, Dorell Wright.

The Portland Trailblazers player didn't play in Kanye's big bash at Staples Center on Monday -- but he was in the building as a celebrity spectator ... and says the most impressive player on the court was JB.

"Bieber was showin' out," Wright said ... "He definitely had the most points. It was between him and Tyga."

Apparently, Kanye wasn't bad either ... but the question -- who wins in a 1-on-1 between Bieber and the birthday boy?

Wright says the answer is pretty obvious ... and it's gonna make one of the guys really mad.