Tommy was a great offensive fighter. If fight were only scheduled for 4 rds he'd never have lost. Unfortunately his chin wasn't all that great and his stamina was suspect too. Eventually he fixed the stamina issues and became slightly more durable. Early in his career and especially in the Mercer fight he would throw everything he had in the first 3 or 4 rds and then get gassed. As soon as he got tired his hands would come down.
Someone mentioned the Lennox Lewis fight.. It was not one of Tommy's better performances. Lewis was had size and stature on Tommy as well as being a better schooled fighter. It was fairly anti climactic. Lewis was just better and an accumulation of punches ended the Duke around the 5th or 6th round.