2021 Western & Southern Open – Men's singles

Alexander Zverev defeated Andrey Rublev in the final, 6–2, 6–3 to win the men's singles tennis title at the 2021 Cincinnati Masters. It was his fifth Masters 1000 title and 17th career singles title on the ATP Tour.

Novak Djokovic was the reigning champion, but did not participate.

Seeds
The top eight seeds receive a bye into the second round. • 🇷🇺 Daniil Medvedev (semifinals)

• Stefanos Tsitsipas (semifinals)

• 🇩🇪 Alexander Zverev (champion)

• 🇷🇺 Andrey Rublev (final)

• 🇮🇹 Matteo Berrettini (third round)

• 🇨🇦 Denis Shapovalov (second round)

• 🇪🇸 Pablo Carreño Busta (quarterfinals)

• 🇳🇴 Casper Ruud (quarterfinals)

• 🇵🇱 Hubert Hurkacz (third round)

• 🇦🇷 Diego Schwartzman (third round)

• 🇮🇹 Jannik Sinner (second round)

• 🇨🇦 Félix Auger-Aliassime (quarterfinals)

• 🇪🇸 Roberto Bautista Agut (first round)

• 🇦🇺 Alex de Minaur (second round)

• 🇧🇪 David Goffin (first round)

• Cristian Garín (first round)

Seeds
• # 🇫🇷 Richard Gasquet (qualified)

• # 🇪🇸 Carlos Alcaraz (qualified)

• # 🇺🇸 Tommy Paul (qualified)

• # 🇯🇵 Yoshihito Nishioka (qualified)

• # 🇩🇪 Dominik Koepfer (qualifying competition, lucky loser)

• # 🇨🇦 Vasek Pospisil (first round)

• # 🇮🇹 Lorenzo Musetti (first round)

• # 🇺🇸 Marcos Giron (qualified)

• # 🇧🇾 Ilya Ivashka (qualifying competition)

• # 🇪🇸 Jaume Munar (qualifying competition)

• # 🇫🇷 Jérémy Chardy (first round)

• # 🇮🇹 Gianluca Mager (first round)

• # 🇫🇮 Emil Ruusuvuori (first round)

• # 🇦🇺 Alexei Popyrin (qualifying competition)

Qualifiers
• # 🇫🇷 Richard Gasquet

• # 🇪🇸 Carlos Alcaraz

• # 🇺🇸 Tommy Paul

• # 🇯🇵 Yoshihito Nishioka

• # 🇺🇸 Marcos Giron

• # 🇫🇷 Corentin Moutet

• # Kevin Anderson

Lucky loser
• # 🇩🇪 Dominik Koepfer