Abstract: Nowadays computer games are being developed for more than just the mere entertainment of the players. In the field of health and education, serious games aim to improve the level of the cognitive abilities, in a more attractive manner. Among the mental process, the concentration and the relaxation are the basic cognitive abilities in tennis. The timely monitoring and the levels of these abilities are important when the performance enhancement is concerned. Mental preparation of tennis players represents an essential aspect of the modern training process. In order to increase its efficiency, the mental preparation uses more often than before biofeedback systems, able to provide an objective and augmented feedback concerning the capacity to control and to maintain the mental abilities at a certain level, during a task. One of the biofeedback systems is represented by Neuroharmony, an electroencephalographic system, which uses also an educational software able to improve the cognitive abilities of the users, through different serious games. In this paper we present one case study concerning the use of Neuroharmony serious games in the mental training of a 16 year old tennis player, who participated in 12 computer assisted lessons. Each lesson lasted 30 minutes and was held twice per week, before the practical training lessons. We were focused on the following experimental variables: the games results and the constancy of the forehand, in a court test. The results reflect some characteristics of the mental training in tennis and the effect of this approach on the practical training efficiency.