Your fatigue determines what Fire Level you can withstand. If you find yourself in a room with a fire level too high for your allowed level, you must deal with the fire level first before doing anything else:
If your Pirate is in a room that has a Fire Level too high for your Fatigue Level, your next action must be to fight the fire, leave the room, or rest until your Fatigue Level is not too high.
If a guard defeats you, they will increase your fatigue. That might increase it to a level where you would otherwise not be allowed to stay in the room without doing the above actions. But when having a rematch against a guard, you can fight the guard first.