Rafael Nadal -- Pumped for Serena & Drake ... He's a 'Great Guy'

Good news for Drake ... he's already getting the big thumbs up from one of Serena Williams' famous pals -- Rafael Nadal -- who tells TMZ Sports the rapper is a "good guy."

We broke the story ... Drake and Serena were all over each other in a Cincinnati restaurant this weekend ... and they'd been flirting pretty hard at some of her tennis matches lately.

Now, it seems Drake's been making the right impression on Serena's contemporaries ... because when Nadal was leaving his NYC hotel yesterday, he made it clear he approved of Drizzy.