They want to lose weight so they're faster and more mobile on the court. That is one way to do it if you're carrying alot of excess fat in the first place. But why would you be holding excess fat if you're a pro athlete? The BEST way to do that is to GAIN weight and muscle. More specifically, bulk up doing low rep squats and deadlifts so you get a huge, massive posterior chain and massive, powerful thighs. And eat a ton of protein and calories. Look at Olympic sprinters, do they have weedy thin legs like tennis players? No, these guys all weigh at least 14st at a height of 5ft 11in. And they're the fastest people in the world. Even huge powerlifters (who carry a lot of fat on them as well) often have blinding 40 yard dash times and vertical leaps of over 35 inches (more than many pro basketball players). Tennis players often weigh as little as 12st if they're as short as 5ft 11in. I'd bet that any elite powerlifter has a faster 40 yard dash and higher vertical leap than any elite tennis player. Tennis is an ANAEROBIC sport with short periods of high intensity followed by rest. Yet most tennis players train like (and look like) distance runners. If they applied anaerobic training principles like low rep weight training I think the game would become faster, more intense and more exciting for the spectator.