I think Hopkins had more energy because he was able to lower Pavlik's punch output. Against Calzaghe, he made it tough for Calzaghe to land cleanly, but Joe was still throwing flurries and letting his hands go. Hopkins had to use up a lot of energy on defense in that fight and that tired him out. Whereas in this fight, he confused and frustrated Kelly so much that Kelly wasn't letting his hands go, so Bernard didn't have to use much energy avoiding/blocking punches.