In tennis, the ATP World Tour Masters 1000 is a term applied to a series of tennis tournaments that are ranked below the four Majors and Year-End Championships. Nonetheless, they are the most prestigious tournaments on the ATP World Tour. The circuit began in 1970, two years after the Open Era when tennis became professional, and has undergone several name changes and slight alterations to the format, in line with overhauls of the sport by the Association of Tennis Professionals.