Yeah. For a fighter who tries to kill his opponents with one shot (which requires a lot of energy), it is quite normal that he would gas out in the latter rounds.
I think he didn't put in too much energy in his shots during the Phil fight, that's why Phil was probably not knocked out (or maybe he has a good chin) and that maybe the reason he didn't gas out in that fight. He looked quite tired getting to the end of the third round though.