Adopting specificity in practice, combined with conditioning, can be an especially effective approach to optimizing training for tennis players. However, little is known about the use of different tennis formats in conditioning drills. The purpose of this study was to compare the effects of singles versus doubles high-intensity interval training (HIIT) tennis training on the aerobic and anaerobic performance of young tennis players, while also evaluating both formats against traditional off-court HIIT training. A randomized parallel controlled study was conducted with 48 male youth tennis players (16.8 ± 0.8 years). The intervention lasted 6 weeks and was carried out twice a week, with singles and doubles HIIT performed in match-play format without serving, while regular HIIT consisted of running (regHIIT). Measurements were taken at baseline and post-intervention, including the Wingate Anaerobic Test (to determine peak and mean power output and decrement), the Hit and Turn Tennis Test (to estimate maximal oxygen uptake, VO_2max), and the 30-15 Intermittent Fitness Test (V_IFT). Using ANCOVA, the Group effect was significant for Wingate peak power. Singles-HIIT exceeded doubles-HIIT (Δ = 0.85 W·kg^-1, 95% CI 0.63-1.08; p < 0.001, Holm), regHIIT (Δ = 0.80 W·kg^-1, 0.58-1.02; p < 0.001), and control (Δ = 1.79 W·kg^-1, 1.57-2.02; p < 0.001); doubles-HIIT and regHIIT did not differ (p = 0.620). For mean power, singles-HIIT was higher than doubles-HIIT (Δ = 1.07 W·kg^-1, 0.80-1.34; p < 0.001), regHIIT (Δ = 1.00 W·kg^-1, 0.74-1.27; p < 0.001), and control (Δ = 1.60 W·kg^-1, 1.33-1.87; p < 0.001); both doubles-HIIT and regHIIT exceeded control (Δ = 0.53-0.60 W·kg^-1; both p < 0.001). VO_2max and VIFT improved similarly across all HIIT formats versus control (VO₂max Δ = 2.10-2.13 mL·kg^-1·min^-1; VIFT Δ = 0.98-1.01 km·h^-1; all p < 0.001), with no differences among HIIT formats (all p ≥ 0.90). In this randomized trial, singles-HIIT produced greater gains in anaerobic performance—showing higher adjusted post-test Wingate peak and mean power—than doubles-HIIT or regHIIT, under the present unmatched-intensity conditions. Aerobic adaptations (VO_2max and VIFT) improved similarly across all HIIT formats and were superior to control. Therefore, singles-HIIT may be preferable when the primary goal is to enhance anaerobic power, whereas any of the three formats can be effectively used for aerobic conditioning, depending on logistical and sport-specific considerations. These inferences apply to the present on-court HIIT protocol in youth players and should be generalized cautiously to other sexes, ages, training levels, and court surfaces.