Many fighters have a common misconception of what should be the most dangerous weapon in their arsenal.
A Brazilian Jiu-jitsu black belt will say that their greatest weapon is their submission ability, where as a boxer or Muay Thai fighter will say it is their knockout ability. A physically muscular fighter will say that their power is their greatest weapon. The list goes on, but what a successful veteran fighter will tell you is that peak cardiovascular shape is their greatest weapon.
Technique, power, speed, and precision are all important keys to winning fights, but what happens when the most powerful or intelligent fighter becomes winded? Everything will fall apart in the cage for the fighter that becomes winded. He might even have been dominating the majority of the fight, but once the gas tank is empty, he is vulnerable to almost anything that the other fighter can throw at him, given the opponent is not gassed as well.
The key to being a successful fighter starts with his physical conditioning. Once this has been established, technique, power, and everything else will fall into shape.