Hmmm...well there is two ways of looking at this.
1. Superman's invulnerability is an aura type thing and the aura has to be overcome before he takes real damage or the strain on the aura exhausts him to the point where he is knocked out.
or
2. Said aura simply enhances his natural invulnerability and he can be knocked out in the traditional manner such as a stiff jab to the chin by someone like Karate Kid.
In scenario 1 Karate Kid could not knock out Superman with his skill because the impact would be distributed across his protective aura which would have to be overcome before Supes would take any real damage; in scenario 2 Karate Kid could knock our Superman with sufficient force because the aura would enhance Superman's resilience but would not leave him vulnerable to a well placed shot to a nerve cluster.