Can the Segunda B minnows upset the Yellow Submarine and etch their name in the Cup's legendary lore?
Match preview
Los Unionistas are no strangers to defying the odds, having clawed their way from the sixth tier of Spanish football to the third tier within 10 years of being established as a club.
Their Copa del Rey journey already boasts 2-0 victories over Segunda B side Gernika and Segunda Division side Sporting Gijon, with the win over the latter in the last round sending shockwaves through Spanish football.
It has been a tough campaign to date so far for the Unionistas, who are 13th in the Primera Federacion Group 1 table with 22 points, boasting a record of five wins, seven draws and six losses from their 18 league matches this season.
However, the home side kickstarted the year positively with a 2-0 win over Barcelona Athletic, which was just the second win in their last 11 league matches.
Manager Dani Ponz will hope home advantage and the electric atmosphere of the Reina Sofía will add another layer of inspiration to their underdog charm on Sunday in what will be their first ever match against the Yellow Submarine.
Villarreal, meanwhile, will enter this match off the back of a 3-1 defeat against Valencia on Tuesday, with Gerard Moreno getting a consolation goal after the Castellon-based side had already conceded three goals before the hour mark.
Marcelino's side have won just five matches, drawn four and lost 10 of their 19 league matches this season sit 13th in La Liga with 19 points, only four points above the relegation zone.
The Yellow Submarine have won just two of their last seven matches in all competitions,and have lost their last two matches on the road.
Villarreal have never won the Copa del Rey, which makes it a priority alongside the Europa League this season.
Marcelino's men will know they are not in for an easy ride on Sunday as it took a goal in extra time to advance past Segunda B side Zamora CF in the previous round.
Unionistas Copa del Rey form:
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Unionistas form (all competitions):
Villarreal Copa del Rey form:
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Villarreal form (all competitions):
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Team News
Unionistas head coach Ponz could name close to the same XI that took to the field for the first whistle against Gijon in the previous round of the competition.
A 4-4-2 formation should include Slavy and Ewan Urain in attack, while Alfred Planas and Adrian Gomez occupy the flanks.
The Yellow Submarine, meanwhile, will be missing a number of players through injury, with Juan Foyth, Yeremi Pino and Denis Suarez unavailable.
Alexander Sorloth and Raul Albiol have to pass late fitness tests for a chance to feature on Sunday, while Aissa Mandi is away on international duty with the Algerian national team.
Unionistas possible starting lineup:
Cacharron; Gimenez, Ramiro, Ruiz, Rojo; Planas, Tur, Rastrojo, Gomez; Urain, Slavy
Villarreal possible starting lineup:
Jorgensen; Moreno, Bailly, Cuenca, Altimira; Capoue, Parejo, Terrats; Akhomach, Morales, Moreno
We say: Unionistas 1-2 Villarreal
Unionistas will put up a good fight fuelled by the home support, but it might not be enough to make up for the gulf in quality between the two sides.