It's hard to lose weight and do competitive workouts (running or tennis) at the same time. I find that it's easier to focus strictly on weight loss while not training for a race or playing tennis matches. I'd guess that there are things that you lose in weight loss mode (vitamins, minerals, carbs, etc.) that you need on training or match-play. Sure, you can manage to get enough of these things but you have to keep track of everything and listen closely and experiment to make sure that you're getting enough of a bunch of things - I find that hard to do.
4 x IG Prestige MP, 70 cm, 376 grams, 386 SW, ALU Power @50