The aim of this study was to examine the differences in linear sprint, change of direction (COD) performance and COD deficit in a large sample of under-13 (U13) and under-15 (U15) male and female tennis players. One hundred and twenty-eight junior tennis players grouped into two age-groups (U13 years (32 males and 28 females) and U15 (36 males and 32 females)) participated in this study. Tests included anthropometric measurements, sprints (5-, 10- and 20-m) and a modified version of the 505 COD test. The differences in performance between age-categories and sex were assessed via an independent t-test. The differences in the physical tests between U13 and U15 players were tested using a univariate analysis, with age and anthropometric variables as covariates. Effect sizes (ES) were calculated for pairwise comparisons. Results showed that males presented lower 20-m sprint times than females in the U13 (ES: 0.54; P< 0.05), and lower linear sprint (5 to 20-m) and COD times than females in the U15 category (ES varying from 0.67 to 1.60; P< 0.05). Comparing age-groups, U15 females demonstrated a higher COD deficit than the U13 (P < 0.05), while no differences were found in the other variables. In males, no significant differences were observed in any variable when comparing both categories. These results could be of great interest for coaches and researchers involved in the development and training of elite tennis players, suggesting the need to include strategies able to improve sprint and COD performance throughout the players’ specialization process.