The tendency in the weight room is to start measuring the length of your johnson by how much you can lift. Leave it to the idiots! If you are training for tennis, you don't want to max out anyway. Few reps and high weight mean bulk and probably some loss of rapid resonse, endurance and flexibility. Everyone in the weight room is different. If lifting 50kg's for 12 reps gives you a good workout, fine. Don't fret that the neanderthal next to you is doing 150kg's once and bragging about it.