There are many ways in which a manager could motivate this sort of an employee.
First, the manager could provide some sort of a financial or other tangible reward. The manager could give the employee a raise, for example. By doing this, they would motivate the employee if that employee is the type that gets their motivation mainly from compensation.
Second, the manager could acknowledge the employee’s good performance both privately and publically. By talking to the worker one on one, the manager could show that they sincerely appreciate the worker’s effort. This might make the worker feel more valued and could motivate them to continue the good work. By praising the worker publically, the manager could improve the worker’s self-esteem and make them feel more important, thus motivating them.
Finally, the manager could give the employee more important work. The manager could conclude that a worker who handles one type of work in an exemplary way might be worthy of more difficult or more important work. Many employees are motivated by the chance to move up in the firm’s hierarchy and by the chance to do more responsible work.
Any of these motivators could also be used in conjunction with one or more of the others. At least one of these ought to work to motivate the high-performing worker.