Tennis is a sport usually played between two or four players using rackets and a ball over a net on various court surfaces. The sport can be played by people of all ages and abilities. Professional tennis players compete in tournaments around the world to earn money from prize winnings rather than salaries. Some of the top professional players today include Roger Federer, Rafael Nadal, Novak Djokovic, Andy Murray, Serena Williams, and Maria Sharapova. Tennis has been an Olympic sport since 1988, with Rafael Nadal and Roger Federer & Stanislas Wawrinka winning gold in recent Games.