Rayo go into the game off the back of an away win over Celta Vigo, which put them level on points with Getafe and Sevilla in eighth place in La Liga, the final position for European qualification.
The Rayo captain told reporters: "We talk about Europe every day in the dressing room and we're going to do everything to make it happen. If we win next Sunday we'll have a much greater chance of achieving it.
"This is one of the most important games of the season because of what we are playing for. They are above us and if we beat them we'll be able to close the gap on them."
The Basques thumped Rayo 4-0 in the previous game between the two sides in November in San Sebastian.
Piti said Sunday's game was a chance to show how his side have developed since that day, but insisted Rayo were not looking at the game as an opportunity to get revenge for that heavy defeat.
"We just want to prove that we are better than we were then and that we have improved as a team," he said.
Real Sociedad defender Inigo Martinez said he was relishing a hard-fought contest between two teams with big ambitions for the remainder of the season.
Philippe Montanier's side currently occupy the fourth Champions League spot and extended their gap over their rivals for that privilege after a resounding 4-2 win over Malaga last weekend.
"The game in Vallecas is as important to us as it is to them and it'll be a real dogfight. We go into the game in a good form and we are fighting to get into the Champions League so it's vital we do a good job there," Martinez told Rayo's official radio station, Union Rayo.
The defender singled out Brazilian forward Leo Baptistao as one of Rayo's danger men but made it clear that he was far from their only threat.
"We know that Rayo are a very strong team and Vallecas has always been a tough place for us to go and of course it's going to be difficult for us to win there," he said.
Rayo have a full squad to choose from for the game, with the exception of Sergio Rodriguez, who has been ruled out for the rest of the season.
Montanier's side, on the other hand, are hopeful that Xabi Prieto, Antoine Griezmann, Carlos Vela and Asier Illaramendi will recover from niggling injuries in time for the game, but midfielder David Zurutuza has been ruled out.
Chory Castro could make his first appearance in five weeks after recovering from abdominal problems.