The 2021 ATP Tour was the global elite men's professional tennis circuit organised by the Association of Tennis Professionals (ATP) for the 2021 tennis season. The 2021 ATP Tour calendar comprised the Grand Slam tournaments (supervised by the International Tennis Federation (ITF)), the ATP Finals, the ATP Tour Masters 1000, the ATP Cup, the ATP Tour 500 series and the ATP Tour 250 series.
https://en.wikipedia.org/wiki/2021_ATP_Tour