Michael Spinks, fast becoming one of boxing's top champions, retained his World Boxing Association light heavyweight title today with an eighth-round knockout of fourth-ranked Murray Sutherland.

Spinks, 172 pounds, opened a two-fisted attack near the middle of the eighth round and staggered Sutherland, 173 3/4, with left hooks and right crosses.

Referee Zack Clayton--with Sutherland cowering under Spinks' battering--rushed in and gave the challenger a standing eight-count.

Sutherland was permitted to continue, but the brief rest did not help. Spinks was immediately on top of him, landing a flurry of punches that culminated with a right uppercut. It dropped the challenger, and he was counted out at 1:24.

Sutherland did not regain his feet for about two minutes. After being taken to his corner, he appeared to be recovering.

Spinks is now 20-0 as a professional, with 14 knockouts. This was his third defense, all ending in knockouts, of the championship he won from Eddie Mustafa Muhammad last July. Sutherland, loser of a close decision to Spinks in 1980 and by TKO to Matthew Saad Muhammad a year ago in a bid for the World Boxing Council title, is 34-5.

Spinks, the younger brother of former heavyweight champion Leon Spinks, who was at ringside in the Playboy Hotel and Casino, took complete command of this bout in the sixth round. Early in the round, the champion snapped Sutherland's head back with a left jab and it became apparent that the challenger was steadily weakening.

Sutherland, who was born in Scotland and lives in Lansing, Mich., managed to land a left-right combination and Spinks waved him on, saying, "Come on, come on." Sutherland obliged and threw a straight right before Spinks launched a hard left hook that wobbled the challenger's knees and followed with a flurry to the head at the end of the round.

Spinks landed a right lead to start the seventh and again Sutherland was covering up and retreating. Two left hooks at the end of the round by Spinks caused severe damage.

Sutherland appeared outmatched almost from the beginning, and his punches seemed to have little power after the third round.