Initially you can challenge it in labour court and later on in High Court. Are you in possession of any letter issued by your employer showing you as under performed?. The employer should have issued you a letter to that effect and the employer should have given you an opportunity to be heard. If this is not followed by the employer and unilaterally takes a stand against your interests, then the same can be challenged in Labour court. Another thing is you can complain about this to the Regional Labour Commissioner.