The Davis Cup is the premier international team event in men's tennis. It is run by the International Tennis Federation (ITF) and is contested annually between teams from competing countries in a knock-out format. It is described by the organisers as the "World Cup of Tennis", and the winners are referred to as the World Champion team. The competition began in 1900 as a challenge between Great Britain and the United States. By 2016, 135 nations entered teams into the competition.