Brief cognitive behavior therapy is planned brief therapy designed to influence a specific target behavior (e.g., sport performance) with the lowest investment of cost and time. We report a brief cognitive behavior therapy intervention delivered to improve serve performance in county-level youth tennis players (three male and one female; Mage = 14.90 years). Following an assessment of athlete needs, we implemented a brief-contact intervention involving diaphragmatic breathing. The intervention was delivered across two “structured” sessions, with the athletes having access to the sport psychologist between sessions as required. To assess the effectiveness of the intervention, we implemented a single-case withdrawal design. Results indicated that the intervention had a positive effect on performance, with athletes’ first-serve percentage increasing significantly (3.91%, p < .05). Results also indicated that a minimum preperformance-routine time (routine > 2 s) might be required for effective performance outcomes. Finally, we offer reflections on the delivery and implementation of a single-case design, while navigating avenues of professional development.