One way to think about it is that they're still HUGE heavyweights. It takes a lot of energy for Fury to box and avoid a big puncher like Wilder. It takes a lot of energy to throw the type of shots Wilder throws. Both men were hurt badly in the first half of the fight, so naturally they're not going to be "fresh" AND they're weary of each other.
It does, but they train for this. Perhaps it is the age showing in Wilder who is otherwise always quite athletic and in shape.
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