Great book about Tennis psychology with a lot of drills. Most of them can be used in a constraints led approach. The following drill sets constraints by changing the rules for counting points. (Miguel Crespo, Machar Reid: Tennis Psychology, 200 + practical drills and the latest research. London 2006)
Players play points from the baseline: -the first point played counts as 1 point -the second and third point count as 2 points each -the fourth and fifth point count as 3 points each -the sixth and the seventh point count as 4 points each -the eighth and ninth points played count as 5 points each -the tenth point played counts as 6 points. Players play 2 sets of 10 points. The same player serves throughout the set.