Before you tackle the problem, remember that if you really want to successfully address the performance issue, it’s critical to maintain a positive attitude towards the employee and not threaten them. If you attack the person they may become defensive with the result that little progress will be made.
Then describe the performance problem in a manner that ensures the employee clearly understands the required level of performance and the areas where their current level of performance is below expectations. Give detailed examples to illustrate your point.
"The KPI for client satisfaction is a rating of 100%. Most other managers regularly achieve that target. From time to time you have also achieved that rating, but the records show that your average is only 81%".
Remember to focus on the performance, not the person. With a performance problem its not a matter of the employee’s attitude, but a matter of assisting a willing employee to perform at an acceptable level.