Frazier had a terrific chin. His style forced him to take far more risks than most. He was legally blind in one eye through out his whole career. Considering these facts and the size of the men he fought I see him as having a great chin.
The reason I mentioned round two of Fight II over those late rounds in Manila is in part due to Frazier corner man Milt Bailey's on camera testimony that both combatants were largely reduced to slaps by the championship rounds in that third brawl. Both were exhausted, and Joe was staggering backwards with fatigue from right hands he couldn't see. But Muhammad did not appear to have retained the kind of late round power he carried in dispatching Bonavena and Wepner by the same stage in Manila. In round two of Fight II, he was obviously at full strength though. Over two and a half years ago, I said in this thread that Tony Perez may well have saved Frazier from a knockdown by his premature gaffe. On review of that footage though, if Ali was ever going to deck Frazier, he would have done it with the first right hand which started that sequence. That shot came close to dropping Joe on his face, but once Frazier recovered his balance and began retreating into a neutral corner, he'd regained a secure defensive posture. (Smoke would have been REALLY pissed and embarrassed if Muhammad had avenged the FOTC knockdown though. Such an occurrence might have turned Fight II into a barn burner comparable to I & III.)
Perfect summary, a clean shot on the chin from a puncher has a good chance of dropping Frazier, but he would keep coming forward absorbing punishment & getting up until you punched him into the grave, if it wasn't for the ref or his corner saving him from himself. One of the most shocking images in boxing is Frazier with his eyes severly swollen & his mouth full of blood, grinning & tapping his chin, inviting Ali to land more punches in Manila. At one point Joe was even tapping his badly swollen eye, encouraging more punishment there! :!: