Everything the other posters have said, especially Rapaire. Of course, the main difference between survival fighting and sport fighting, is that the latter is deliberately structured to give both sides an even chance and to reduce the prospect of injury.
The phrase "fair fight" is an oxymoron on a level with "beautiful orc" or "virgin whore". There is no such thing as a "fair fight".
Remember, if you have an attacker's hand on you, your whole hand is stronger than a single finger. Do not hesitate to break or dislocate a finger. You ain't arm wrestling, you are fighting. Bite anything within reach of your teeth. If you get a shot at getting a blow in, good targets are the Adam's apple, or just below it, the soft part under the chin (a thumb will do the trick if it is jerked hard), the fleshy part of the armpits, the pressure points at the side of the neck, the temples (if hit with a knuckle on each side) the pressure points between the skull and the top of the jawbone (just below the earlobe). Another soft target is the area between the hip bone and the bottom of the rib cage.If you are attacked from behind, you may get lucky and catch that with an elbow. As Rapaire says, if you hit it with something hard, so much the better. Do any of that in karate and you will get banned for life. In a street fight you will not be asking for a rematch. If you do get knocked down, try to keep your assailant at bay with your feet. Your legs are stronger than your arms, so scream and make a lot of noise, while using short, snappy kicks, to buy you time.
Best of all, get the hell out of danger before a fight even starts. The best fights are the ones you never get in.