Thing is the very best tried to KO Ali and they all failed (of course discounting Holmes). With this in mind I don't believe anyone kos prime Ali. Too good and too tough. However the punch to hit Ali with was the left hook and Joe had a hard and fast one. He hits prime Ali with this blow and he hurts him and possibly knocks Ali down. However Ali's boxing ability wins out and his chin gets him through the rough spots. Ali 15 round decision in a 9-6 type bout.
I think Frazier could pressure him and keep him on the ropes enough to get a decision. The 64 Cassius Clay is not the same as the 67 Muhammad Ali.