Klay Thompson was PISSED after his friend got fouled hard in a pick-up game in China -- with the NBA star cussing out one of the players and nearly coming to blows.
What set Klay off was when Klay's pal drove to the hoop and an opposing player took out his legs in midair -- causing the guys to come crashing down HARD on the floor.
Klay's pal was rightfully furious -- and that's when Thompson stormed onto the court to get in the other dude's face.
"Come on, man. That's bullsh*t," Thompson says.