2021 National Bank Open – Men's singles

Daniil Medvedev defeated Reilly Opelka in the final, 6–4, 6–3 to win the men's singles tennis title at the 2021 Canadian Open. It was his fourth ATP Masters 1000 title and 12th career title. Opelka was contesting his maiden Masters 1000 final.

Rafael Nadal was the two-time reigning champion from when the event was last held in 2019 and was vying to tie Ivan Lendl's record of six Canadian Open titles, but did not participate due to a foot injury.

Seeds
All seeds received a bye into the second round. • 🇷🇺 Daniil Medvedev (champion)

• 🇪🇸 Rafael Nadal (withdrew due to left foot injury)

• Stefanos Tsitsipas (semifinals)

• 🇷🇺 Andrey Rublev (third round)

• 🇨🇦 Denis Shapovalov (second round)

• 🇳🇴 Casper Ruud (quarterfinals)

• 🇵🇱 Hubert Hurkacz (quarterfinals)

• 🇦🇷 Diego Schwartzman (third round)

• 🇨🇦 Félix Auger-Aliassime (second round)

• 🇪🇸 Roberto Bautista Agut (quarterfinals)

• 🇫🇷 Gaël Monfils (quarterfinals)

• 🇦🇺 Alex de Minaur (second round)

•  Cristian Garín (second round)

• 🇧🇬 Grigor Dimitrov (second round)

• 🇷🇺 Aslan Karatsev (second round)

• 🇮🇹 Jannik Sinner (second round)

Seeds
• # 🇺🇸 Frances Tiafoe (qualifying competition, lucky loser)

• # 🇺🇸 Tommy Paul (qualified)

• # 🇯🇵 Yoshihito Nishioka (qualified)

• # 🇩🇪 Dominik Koepfer (first round)

• # 🇮🇹 Lorenzo Musetti (removed for exiting Covid-19 bubble)

• # 🇺🇸 Marcos Giron (first round)

• # 🇫🇮 Emil Ruusuvuori (qualified)

• # 🇦🇺 Alexei Popyrin (first round)

• # 🇦🇺 James Duckworth (qualified)

• # 🇱🇹 Ričardas Berankis (qualified)

• # 🇪🇸 Feliciano López (qualifying competition, lucky loser)

• # 🇫🇷 Benjamin Bonzi (first round)

Qualifiers
• # 🇫🇮 Emil Ruusuvuori

• # 🇺🇸 Tommy Paul

• # 🇯🇵 Yoshihito Nishioka

• # 🇦🇺 James Duckworth

• # 🇱🇹 Ričardas Berankis

• # 🇨🇦 Brayden Schnur

Lucky losers
• # 🇺🇸 Frances Tiafoe

• # 🇪🇸 Feliciano López