Common is strongly siding with his ex-GF, Serena Williams -- telling TMZ Sports someone is trying to "take her down" by drug testing her more than usual ... and it's all because she's a black woman.
Hours before we spoke with Common, Serena went off about being drug tested more than any other female tennis player ... and accused officials of "discrimination."
Common clearly agrees -- saying, "As a black woman, do you think she's getting discriminated against? YES."
"If they're testing her more than anyone else, it's a reason for that.