## 1 Introduction  
Sports can be mentally as well as physically firing [1]. Athletes experience mental fatigue due to cognitive efforts associated with technical-tactical decisions and emotion regulation. In addition, extrinsic variables such as travel or studies can contribute to mental fatigue accumulation [2; 3]. Mental fatigue is characterized by subjective feelings of tiredness, exhaustion, aversion to continuing the current activity, or a decrease in perceived commitment during or after a task. The objective changes induced by these feelings are referred to as fatigability, which is associated with impaired cognitive and behavioral performance [4; 5].  
Previous studies have investigated the impact of physical and mental tasks on the perception of fatigue and fatigability. One consistent finding is the increase in the rate of perceived exertion (RPE) during exercise. It can be explained by alterations in the processing of neural signals underlying the perception of effort [6; 7; 8; 9]. Questionnaires and scales, such as the NASA Task Load Index or visual analog scales (VAS), have been used to evaluate subjective feelings of mental fatigue [10; 11].  
Regarding fatigability, research has examined the effects of mental fatigue on various performance outputs. Aerobic endurance performance has shown significant impairments, including decreased time to exhaustion, self-selected velocity, and increased completion time [11]. However, negative impairments have not been consistently observed in anaerobic tests due to the short duration of the efforts [11; 12; 13]. Physiological variables such as heart rate, blood lactate, and cardiac output have generally been unaffected by mental fatigue. Nevertheless, the impact of mental fatigue on heart rate variability (HRV) has not been extensively explored. HRV, specifically the root mean square of successive differences (RMSSD), is considered a biomarker of fatigue and has shown sensitivity to cognitive stimuli [14; 15; 16; 17; 18].  
Many sports, including team or racket sports, involve a combination of aerobic and anaerobic efforts. Previous studies have demonstrated that mental fatigue can negatively affect physical sports such as soccer or basketball, where the aerobic and anaerobic systems are utilized. High-intensity interval training (HIIT) is a well-known training method in these sports, combining short anaerobic efforts with brief periods of aerobic recovery. HIIT has shown benefits for physical conditioning and mental health [19; 20; 21; 22; 23].  
Recent studies have examined the effects of combining HIIT with a cognitive task, particularly the incongruent condition of the Stroop color and word test (SCWT), which induces mental fatigue [24; 25; 26]. The SCWT involves selective attention, interference inhibition, and processing speed. A previous study found a significant reduction in accuracy in tennis serves after HIIT combined with a cognitive task [24]. However, the impact of HIIT combined with a cognitive task on RMSSD and subjective mental fatigue has not been extensively explored. Furthermore, most studies have induced mental fatigue before physical load with cognitive activities longer than 30 min. However, mental fatigue and fatigability in sports typically occur during matches or competitions such as tennis, where tennis players often experience challenging environmental conditions [1; 27] and physical demands that require high-intensity interval efforts.  
Therefore, and taking into account previous research in the field that suggested that shorter tasks could induce mental fatigue in sports [1], the present study aims to investigate the effect of inducing mental fatigue on HIIT in a sample of tennis practitioners. We hypothesize that mental fatigue will negatively influence the behavioral outcomes (mean and maximum heart rate and speed), perception of effort and mental fatigue, muscle soreness, stress level, mood state, and RMSSD of tennis practitioners during HIIT.