Tennis is an action sports. While your photos are sharp and nicely exposed, they don't convey action. You need to set the shutter speed such that the racket and the ball are somewhat blurred upon impact (not always, but in general and as an example). I used to shoot quite a bit of tennis, but that was so long ago I don't remember the setting that I used. One thing that helps a great deal is to preview in advance those tennis photos (from tennis magazine or Internet) shot by pros who make living doing that sort of thing. Your photos are nice, though. They just need to convey some action drama rather than tennis "posing."