Changing constraints like court size, balls, rules, equipment, court surface, interference, improves the evolution of your players game. Working differencial with regularly changing constraints without repetition gives players the chance to find creative solutions for unexpected situations of the game. Setting this in an playful approach makes motor learning and skill development more creativ and stable.
In this example players have to work on service and return. It is not surprising, that players start in this drill with more double-faults than expected. After a while, when successful solutions occure, change constraints by using variations.