Rafael Nadal says self-doubt and competition pushed him to be a better tennis player. Joe Portlock/Getty Images Rafael Nadal says having self-doubt contributed to his success in tennis. "The doubts are positive because they allow me to go on court knowing that I need to improve," he said. He said his rivalries with Roger Federer and Novak Djokovic also pushed him to get better at the sport. Rafael Nadal, 40, says constant self-doubt and competing against tennis's greatest players made him a be