This study examines the impact of cognitive load (CL) on lower limb biomechanics during the take-off preparation phase of the platform serve in tennis players of varying skill levels. Fifteen elite and fifteen amateur tennis players performed platform serves before and after completing a 30-minute Stroop task designed to induce CL. Key biomechanical parameters, including joint range of motion (ROM), joint moments, centre of mass (COM) displacement, and ground reaction force (GRF), were assessed using both kinematic and kinetic analysis. After CL, the biomechanical performance of amateur athletes significantly decreased compared to elite athletes. Specifically, amateur athletes showed a 14.19° lower ankle joint range of motion in the sagittal plane (p < 0.001), a 0.04-meter lower COM displacement in the frontal plane (p = 0.017), and a 0.15 Nm/BW lower knee extension moment (p < 0.001). CL adversely affects the lower limb biomechanics of amateur players more than elite players, with elite players demonstrating greater stability. These findings suggest that elite players have developed more efficient motor control mechanisms through extensive training. Tailored training interventions that account for different skill levels could enhance performance stability and mitigate the risk of injury.