2021 Kremlin Cup – Men's singles

Andrey Rublev was the reigning champion from when the tournament was last held in 2019, but lost to Adrian Mannarino in the second round in a rematch of the 2019 final.

Aslan Karatsev won the title, defeating Marin Čilić in the final, 6–2, 6–4.

Seeds
The top four seeds received a bye into the second round. • #🇷🇺 Andrey Rublev (second round)

• #🇷🇺 Aslan Karatsev (champion)

• #🇷🇺 Karen Khachanov (semifinals)

• #🇷🇸 Filip Krajinović (second round)

• #🇰🇿 Alexander Bublik (first round)

• # Marin Čilić (final)

• #🇧🇾 Ilya Ivashka (withdrew)

• #🇷🇸 Laslo Đere (first round)

Seeds
• # 🇧🇾 Egor Gerasimov (qualified)

• # 🇱🇹 Ričardas Berankis (qualifying competition, lucky loser)

• # 🇲🇩 Radu Albot (first round)

• # 🇧🇦 Damir Džumhur (qualified)

• # 🇦🇹 Jurij Rodionov (qualifying competition)

• # 🇨🇿 Zdeněk Kolář (qualifying competition)

• # 🇦🇺 Marc Polmans (first round)

• # 🇸🇪 Elias Ymer (qualifying competition)

Qualifiers
• # 🇧🇾 Egor Gerasimov

• # Borna Gojo

• # 🇺🇦 Illya Marchenko

• # 🇧🇦 Damir Džumhur

Lucky loser
• # 🇱🇹 Ričardas Berankis