If you want to excel at tennis and make it more than just a hobby, you need to become an expert. Now, very few individuals actually become experts in anything. There simply is always more to learn and more to uncover. It simply means you need to do your part to strive towards excellence and do everything in your power to become better than those around you and, by doing so, become an expert.
During every practice, you need to have a reason. Don't just show up, smack the ball around for an hour or two and go home. That doesn't do you much good. You need a deliberate area to practice on. Focus on a specific. You need to want to make yourself better than you were the previous day. Maybe you focus on your grip, or the amount of backspin you place on a ball. Perhaps it is footwork. Whatever it is, practice deliberately. Your competition will focus on things they already do well, you need to focus on what you don't. Your tennis game is only as good as the weakest link, so strengthen that weakest link.
It's all about time
Put in deliberate effort towards making your tennis game better. If you ever watch a sporting event on television, chances are you'll hear the announcer talk about how the player used to shoot 1,000 shots on their own after practice, or they took batting practice for hours at a time. It doesn't matter the sport, it simply shows that to excel and become an expert, you need to put in the time and go above and beyond what everyone else does. You want to be great? You need to do things other people won't do. So it just rained and the court is a bit wet? Everyone else is inside, yet you should be practicing. Even if you can't actually practice with a ball, practice your footwork. Greatness comes from dedication and the time you put into it. The only way you'll ever become an expert in tennis is if you devote yourself to it.