Turns out, Draymond Green's a big tennis fan -- but the NBA star says he was so upset when Serena Williams got bounced from the U.S. Open last night, he won't watch the rest of the tourney.
"I'm mad Serena lost. I hate to see Serena lose!"
FYI, Green and Odell Beckham were in the stands when Serena beat Simona Halep on Wednesday ... and thought she'd coast over 10th seed Karolina Pliskova on Thursday night.
But Pliskova shocked the world ... and Draymond ain't happy about it.
"Ain't your day every day," Green said ... "Can't win 'em all. She's still the greatest."