I assume you talk about orc patrols near Reddock.
After you knocked down one orc, while focusing on it, press and hold right mouse button, and then press left mouse button.

Most humanoid opponents in the game (humans and orcs) are only fought to KO, not death. This works for you as well- if you lose they'll only knock you down then take some of your money. If you want to kill such an opponent then follow the instructions kojocel provided. However, there are some exceptions to this rule. If an enemy's name is displayed in red (rather than yellow or white) then this means it's a fight to the death, for both of you. Some enemies in the game are like this by default (orcs that are automatically hostile), and others will become like this if you perform certain actions (such as killing a KOed opponent in front of his buddies). Also, all of this only applies to melee attacks; ranged attacks and magic always kill.