Klay Thompson was PISSED after his friend got fouled hard in a pickup 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.
"If you want to fight him, then fight his ass -- don't undercut him. F*ckin' bullsh*t. That's bullsh*t, man. Bullsh*t. You don't do that bullsh*t."
"That's how you f*ckin' ruin somebody's career, god damnit. You don't do that. You don't do that sh*t."
Cooler heads prevailed, and Thompson left the court ... but #ChinaKlay's quickly taking on a whole new meaning this summer.