2021 Open de Oeiras III

Country	Player	Rank1	Seed
CZE	Jiří Veselý	73	1
USA	Steve Johnson	85	2
ARG	Federico Coria	91	3
ESP	Pedro Martínez	94	4
GER	Yannick Hanfmann	95	5
ARG	Juan Ignacio Londero	98	6
ARG	Facundo Bagnis	105	7
POR	Pedro Sousa	111	8

• ¹ Rankings are as of 10 May 2021.^[1]

The following players received wildcards into the singles main draw:

• Nuno Borges
• João Domingues
• Gastão Elias

The following player received entry into the singles main draw as an alternate:

• Frederico Ferreira Silva

The following players received entry from the qualifying draw:

• Zizou Bergs
• Emilio Gómez
• Hugo Grenier
• Alex Molčan

• Carlos Alcaraz def. Facundo Bagnis 6–4, 6–4.

• Hunter Reese / Sem Verbeek def. Sadio Doumbia / Fabien Reboul 4–6, 6–4, [10–7].