Background: The importance of the serve in tennis players’ performance is well known but no previous studies have analyzed the effects of both physical and cognitive fatigue on the speed and accuracy of the serve. This study analyzed the effect of a High-Intensity Interval Training (HIIT) with and without cognitive load on serve speed and accuracy, spirometry, and strength manifestation. Methods: 32 recreational players (25 men and 7 women; aged 21.40 ± 1.52 years) performed a HIIT and a HIIT with a Stroop in recovery phases before performing a series of tennis services. Speed and accuracy of the services, spirometry, and strength manifestations were registered. Results: The main findings of the study showed that strength manifestations and spirometry were not affected by either protocol. A decrease in serve speed was observed in both protocols (p < 0.001) but service accuracy did not show impairments (p = 0.66). Conclusion: A combination of physical and mental fatigue may decrease serve speed but will not affect strength manifestations or spirometry negatively. These results could be caused by a response of the central nervous system to maintain the accuracy of the ball in presence of fatigue.