2018 Croatia Open Umag – Singles

Andrey Rublev was the defending champion, but lost in the quarterfinals to Robin Haase. This was Rublev's first tournament back on tour after missing three months due to a stress fracture in his back.

Marco Cecchinato won the title, defeating Guido Pella in the final, 6–2, 7–6(7–4).

Seeds
The top four seeds receive a bye into the second round. • #🇬🇧 Kyle Edmund (withdrew due to a throat infection)

• #🇧🇦 Damir Džumhur (second round)

• #🇮🇹 Marco Cecchinato (champion)

• #🇷🇺 Andrey Rublev (quarterfinals)

• #🇪🇸 Albert Ramos Viñolas (second round)

• # Robin Haase (semifinals)

• # João Sousa (first round)

• #🇫🇷 Benoît Paire (first round)

• #🇩🇪 Maximilian Marterer (second round)

Seeds
• # 🇸🇰 Martin Kližan (qualified)

• # 🇧🇷 Rogério Dutra Silva (qualified)

• # 🇷🇺 Alexey Vatutin (qualifying competition, retired)

• # 🇮🇹 Stefano Travaglia (qualified)

• # 🇦🇷 Facundo Bagnis (first round)

• # 🇪🇸 Enrique López Pérez (first round, retired)

• # 🇸🇰 Andrej Martin (qualifying competition, lucky loser)

• # 🇫🇷 Kenny de Schepper (qualifying competition)

Qualifiers
• # 🇸🇰 Martin Kližan

• # 🇧🇷 Rogério Dutra Silva

• # 🇦🇷 Marco Trungelliti

• # 🇮🇹 Stefano Travaglia

Lucky loser

 * 1) 🇸🇰 Andrej Martin