The Game thinks Serena Williams is an AMAZING rapper and if she decides to drop an album, it's sure to sell big ... this according to the man himself.

Game tells TMZ ...  after he heard Serena's rap for the first time, he knew that she definitely had "raw skills" and was a "really talented artist."

Game says he wash't that surprised ...  considering the tennis champ's Compton roots. He tells us "You shouldn’t expect her to come with anything less than the official rap bars."

Hate it or love it ... looks like Serena's rap skills are here to stay.