The experimental blended group (perform-up group) used the application Perform-UP Tennis on their own and also met up with a professional sports psychologist every 2 weeks for debriefing. Participants were asked to answer a series of questionnaires in two separate phases.

Participants were asked to answer a series of questionnaires in two separate phases, but they only carried out their classic technical training without integrating the mental training program with the app Perform-UP Tennis.

Perform-UP Tennis is a sport specific mental training app that integrates breathing, relaxation and na-ture-based guided imagery exercises and aims to promote emotional well-being and the enhancement of mental skills. The app techniques are integrated into a single gradual path that involves increasingly complex breathing and relaxation exercises with increasing duration.

The intervention consists of eight weekly modules over a period of 8 weeks with progressive exercises.

Each scale included five items, to which the athletes responded on a 5-point Likert scale. The eight subscales reflected the following eight mental abilities: concentration, arousal control, preparation of the match, goal setting, visualization, cognitive anxiety, self-confidence and self-talk. But for this research, the investigators considered only three subscales that were consistent with the activities proposed within the application (breathing and relaxation techniques). The scales considered were: cognitive anxiety, which evaluates the athlete's level of concern during the match, fear of making mistakes, and fear of failing, self-confidence, which evaluates the confidence that the athlete has in being able to compete at his/her best, to give their best and to believe in themselves, emotional arousal control, which evaluates the athlete's ability to relax when he/she feels anxious and under tension and to activate him/herself when he/she needs to reach the right energy level.

Each scale included five items, to which the athletes responded on a 5-point Likert scale. It contains both positively and negatively worded items (the non-judgmental subscale was reverse-scored). Specifically, this questionnaire assessed: (1) Awareness, being aware of stimuli and their associated internal reactions, (2) non-judgmental, adopting a non-judgmental attitude towards these stimuli and reactions, (3) refocusing, quickly refocus attention on target signals.