In tennis, the ATP Masters is an annual series of nine top-level tennis tournaments featuring the elite men's tennis players on the ATP Tour. The tournaments are important for the top players on the professional circuit as the series constituted the most prestigious tournaments in men's tennis after the four Grand Slam events. The Masters series along with the Grand Slam tournaments, the ATP Finals championship and the Olympic Games are considered the top-tier events of men's tennis, referred to by the ATP as the "Big Titles".