The 2021 San Diego Open was a men's tennis tournament played on outdoor hard courts. It was the 1st edition of the San Diego Open, and part of the ATP Tour 250 series of the 2021 ATP Tour.
It was primarily organized due to the cancellation of Asian tournaments during the 2021 season, because of the ongoing COVID-19 pandemic. Positioned one week before the 1000 Indian Wells Masters and only a 2-hour 30 min drive from Indian Wells, the San Diego Open attracted a very strong line up of players including Denis Shapovalov, Andrey Rublev, Grigor Dimitrov and Andy Murray.[1] With 8 of the top 20 players attending and the ranking cut off being 42 to get into the main draw it was the second most competitive ATP 250 all season since Doha.
It was held at the Barnes Tennis Center in San Diego, United States, from September 27 until October 3, 2021. Daniel Vallverdu took on the position of managing director,[2] Ryan Redondo[3] was named the tournament director and Billie Jean King[4] accepted the role of honorary tournament chair.
After beating Dimitrov in the semifinal Casper Ruud went on to win the final of the 2021 San Diego Open beating Cameron Norrie 6-0 6-2 and was handed the trophy by Rod Laver. It was Ruud's fifth victory of the season.
See main article: 2021 San Diego Open – Singles.
Casper Ruud def. 
Cameron Norrie, 6–0, 6–2See main article: 2021 San Diego Open – Doubles.
Joe Salisbury / Neal Skupski def. 
John Peers / 
Filip Polášek, 7–6(7–2), 3–6, [10–5]| Country | Player | Rank1 | Seed | |
|---|---|---|---|---|
| Andrey Rublev | 5 | 1 | ||
| Norway | Casper Ruud | 10 | 2 | |
 Canada | ||||
| Canada | Denis Shapovalov | 12 | 4 | |
 Poland | Hubert Hurkacz | 13 | 5 | |
 Argentina | Diego Schwartzman | 15 | 6 | |
| United Kingdom | Dan Evans | 23 | 8 | |
 Italy | Lorenzo Sonego | 24 | 9 |
The following players received wildcards into the main draw:
The following player received entry as a special exempt:
The following players received entry from the qualifying draw:
The following players received entry as lucky losers:
Félix Auger-Aliassime → replaced by August Holmgren
David Goffin → replaced by 
Sebastian Korda
Kwon Soon-woo → replaced by Kevin Anderson
Dušan Lajović → replaced by Lloyd Harris
Kei Nishikori → replaced by Denis Kudla Reilly Opelka → replaced by Federico Delbonis| Country | Player | Country | Player | Rank1 | Seed | |
|---|---|---|---|---|---|---|
| United Kingdom | Joe Salisbury | United Kingdom | Neal Skupski | 25 | 1 | |
| United Kingdom | Jamie Murray |  Brazil | Bruno Soares | 29 | 2 | |
| Australia | John Peers | Slovakia | Filip Polášek | 33 | 3 | |
| Italy | Simone Bolelli | Argentina | Máximo González | 57 | 4 |
The following pairs received wildcards into the doubles main draw:
Nicholas Monroe / Frances Tiafoe → replaced by Dominik Koepfer / Nicholas Monroe Rajeev Ram / Jack Sock → replaced by Jordan Thompson / Jackson Withrow