Fight one more round.
When your feet are so tired that you have to shuffle back to the centre of the ring, fight one more round.
When your arms are so tired that you can hardly lift your hands to come on guard, fight one more round.
When your nose is bleeding and your eyes are black and you are so tired you wish your opponent would crack you one on the jaw and put you to sleep, fight one more round...
...remembering that the man who always fights one more round is never whipped.