2021 Belgrade Open – Singles

This was the first edition of the tournament, primarily created due to the one-week delay of the 2021 French Open.

Novak Djokovic won the title, defeating Alex Molčan in the final, 6–4, 6–3. It was Djokovic's third title ever in a tournament held in Serbia (excluding the Davis Cup).

Seeds
The top four seeds receive a bye into the second round. • # 🇷🇸 Novak Djokovic (champion)

• # 🇫🇷 Gaël Monfils (second round)

• # Nikoloz Basilashvili (second round)

• # 🇫🇷 Adrian Mannarino (second round)

• # 🇷🇸 Dušan Lajović (quarterfinals)

• # 🇷🇸 Filip Krajinović (withdrew)

• # 🇷🇸 Miomir Kecmanović (first round)

• # 🇦🇷 Federico Delbonis (semifinals)

Seeds
• # 🇦🇷 Federico Coria (moved to main draw)

• # 🇪🇸 Roberto Carballés Baena (qualified)

• # 🇸🇰 Andrej Martin (qualified)

• # 🇦🇺 Christopher O'Connell (qualified)

• # 🇩🇪 Mats Moraing (qualifying competition, lucky loser)

• # 🇸🇰 Lukáš Klein (qualifying competition, lucky loser)

• # 🇨🇿 Vít Kopřiva (qualifying competition)

• # 🇸🇰 Alex Molčan (qualified)

Qualifiers
• # 🇸🇰 Alex Molčan

• # 🇪🇸 Roberto Carballés Baena

• # 🇸🇰 Andrej Martin

• # 🇦🇺 Christopher O'Connell

Lucky losers
• # 🇩🇪 Mats Moraing

• # 🇸🇰 Lukáš Klein