Also included in the 2021 calendar are the Summer Olympic Games, which were rescheduled from 2020.

[1] The COVID-19 pandemic affected tournaments on both the ATP and WTA tours.

The players/nations are sorted by: The following players won their first main circuit title in singles, doubles, or mixed doubles: The following players defended a main circuit title in singles, doubles, or mixed doubles: The following players achieved their career high ranking in this season inside top 50 (in bold the players who entered the top 10 for the first time).

* Assumes undefeated round robin match record.

The following is a list of notable players (winners of a main tour title, and/or part of the WTA rankings top 100 in singles, or top 100 in doubles, for at least one week) who returned from retirement or inactivity during the 2021 season: The following is a list of notable players (winners of a main tour title, and/or part of the WTA rankings top 100 in singles, or top 100 in doubles, for at least one week) who announced their retirement from professional tennis, became inactive (after not playing for more than 52 weeks), or were permanently banned from playing, during the 2021 season: Women's World Tennis circuit|1923]]