Russia's Gradovich won the bout 114-112 in the eyes of two judges, while another had Dibb of Australia ahead 114-112.
Both fighters were deducted a point by referee Eddie Claudio for excessive holding in the eighth of 12 rounds.
In a scrappy encounter, Dib was cut by an accidental elbow early on and ended the fight with blood pouring from around his right eye.
Victory for Gradovich takes his record to 16-0 with eight KOs, while Dib falls to 35-2, with 21 KOs, after his first defeat for four-and-a-half years.
