Rampage had to go through those same fights like Liddell would have so it isnt so much the story of gassing but the fact that Rampage could overpower Liddell too. And if memory serves he stunned Liddell a few times with his own punches. And I think one of the main reasons Liddell got tired was trying to get up when Rampage put him down just because of the sheer strength of Jackson it nearly took all Liddell had just to get out of it, it didn't help that Rampage was kneeing him in the face everytime he tried to get up so by the time Rampage took Liddell down again he didn't have anymore.

Rampage aint been the same fighter since the second fight with Silva unfortunately