Start the game and wait for the boxing to begin. Now press ‘P’ to pause. Press ‘F10’ six times and then press the fire button to unpause. You will now have infinite energy.
If you are about to knock out your opponent but there is less than 10 seconds left on the clock, back away and let the round end. If you knock the opponent down, the round will end while the ref counts to 10, and the opponent will get back to his feet regaining 25% of his energy. Then both players will get another 25% energy back for starting the next round. If you wait until the next round starts, you will only have to knock off 25% of his energy to knock him over.