Since 1990, the biggest events in men's tennis have been the four Grand Slam tournaments, the ATP Finals and the ATP Masters tournaments, in addition to the Grand Slam Cup between 1990–99. From 1983 to 1990, men's tennis had a very strong tradition and clear hierarchy of tournaments: the Grand Slam tournaments, including Wimbledon, the US Open, the French Open, and the Australian Open; the season-ending Masters Grand Prix; and the Davis Cup. Before 1983, however, and in particular before the start of the Open Era in 1968, the hierarchy of professional tournaments changed virtually every year. For example, in 1934, the U.S. Pro was a high-class tournament with all the best players, but just two years later, the same tournament was ordinary because only professional teachers (no leading tour