Tennis serve is a closed-skill motion influenced by the kinematic chain concept. An important role on the final performance is surely linked to the knee joint motion, although only few studies addressed lower-limb influence. In this work we propose a study about the proprioceptive angle size of knee joint in the first flat serve produced by tennis players of different performance levels. Twelve subjects involved in the study are divided in three groups using the ITN classification code: beginner, intermediate and advanced players. A simple biomechanical model of lower limb was proposed to study the knee joint motion. Three flat serves were recorded using three synchronized cams. A specific algorithm was developed to obtain the maximum flexion value of front knee using coordinates of lower-limb joints. Speed of impact (S B ) and height of impact (H B )were also evaluated to compare results with previous works and to assess the tennis players' performance. Results showed a statistical meaningful difference among three groups in all performance indexes. Moreover a strong correlation was found among knee motion and S B and H B .