2021 Open de Oeiras IV

Country	Player	Rank1	Seed
FRA	Mathias Bourgue	195	1
TPE	Wu Tung-lin	267	2
GER	Julian Lenz	269	3
TPE	Tseng Chun-hsin	274	4
ECU	Roberto Quiroz	275	5
ESP	Carlos Gimeno Valero	286	6
ARG	Pedro Cachin	292	7
ITA	Riccardo Bonadio	293	8

• ¹ Rankings are as of 17 May 2021.^[1]

The following players received wildcards into the singles main draw:

• Pedro Araújo
• Tiago Cação
• Luís Faria

The following player received entry into the singles main draw using a protected ranking:

• Viktor Galović

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

• Corentin Denolly

The following players received entry from the qualifying draw:

• Filip Cristian Jianu
• Nikolás Sánchez Izquierdo
• Timofey Skatov
• Denis Yevseyev

• Gastão Elias def. Holger Rune 5–7, 6–4, 6–4.

• Jesper de Jong / Tim van Rijthoven def. Julian Lenz / Roberto Quiroz 6–1, 7–6^(7–3).