OBJECTIVE To compare the effects of combining high-intensity training (HIT) and sport-specific drill training (MT) versus sportspecific drill training alone (DT) on fitness performance characteristics in young tennis players. METHODS Twenty young tennis players (14.8 ± 0.1 y) were assigned to either DT (n = 10) or MT (n = 10) for 8 wk. Tennis drills consisted of two 16- to 22-min on-court exercise sessions separated by 3 min of passive rest, while MT consisted of 1 sport-specific DT session and 1 HIT session, using 16-22 min of runs at intensities (90-95%) related to the velocity obtained in the 30-15 Intermittent Fitness Test (VIFT) separated by 3 min of passive rest. Pre- and posttests included peak oxygen uptake (VO2peak), VIFT, speed (20 m, with 5- and 10-m splits), 505 Agility Test, and countermovement jump (CMJ). RESULTS There were significant improvements after the training period in VO2peak (DT 2.4%, ES = moderate; MT 4.2%, ES = large) and VIFT (DT 2.2%, ES = small; MT 6.3%, ES = large) for both DT and MT, with no differences between training protocols. Results also showed a large increase in the 505 Agility Test after MT, while no changes were reported in the other tests (sprint and CMJ), either for MT or DT. CONCLUSIONS Even though both training programs resulted in significant improvements in aerobic performance, a mixed program combining tennis drills and runs based on the VIFT led to greater gains and should be considered the preferred training method for improving aerobic power in young athletes.