Just believe in yourself. Even if you don't pretend that you do and, and some point, you will.

—Venus Williams

Venus Williams is one of the most famous tennis players of all time. She has four Olympic gold medals and one silver medal, 7 Grand Slam singles titles, 14 Grand Slam doubles titles with her sister, Serena Williams, and 2 mixed Grand Slams. She has also won 49 WTA singles titles and 22 WTA doubles and has been ranked WTA World Number 1 three times in her career. All in all, you could say Williams is pretty good at tennis. 

You don't get good at your sport by only training for your sport. You have to train your mind as well. It will be hard at first, but you need to have it set in your mind that you can do great things. It doesn't matter how many people don't believe in you and tell you that you can't do it. The only person that has to believe in you is yourself. 

Tennis is a very mental game. You make one mistake and your mindset can change in one second. You have to understand that every game is going to come with mistakes. However, to be exceptional in your sport, you have to be able to shake those mistakes on and keep believing in yourself. Even if you feel like you can, just do what Venus Williams says, and fake it until you make it.