But they can change the scheduled time of their tests? IDK how much prior notice they get for those, so maybe being able to reschedule doesn't matter.
I think testing them after matches and randomly throughout the year should catch and prevent both doping that helps during matches and training/offseason doping for strength or whatever else they would use it for. Doping for tennis is more about endurance and stamina than strength, so it needs to be working while they're playing and should be detectable right after a match.