Barcelona and Athletic Bilbao, the two most successful teams in the history of the Copa del Rey, will meet Tuesday in the round of 16 in a repeat of the 2009 final.
Bilbao will play the first leg away at Camp Nou stadium after having won its past two games, but will be without injured star striker Fernando Llorente.
Barcelona won the final two years ago to claim a record 25th Copa title, two more than Bilbao.
Also Tuesday, Valencia and Villarreal meet in a provincial derby.
The loss of Llorente because of a pulled muscle will undoubtedly be felt by Bilbao. The Spanish international has scored 10 goals in the league this season and is the focus of his team's attack.
"He (Llorente) is a focal point for them, but Athletic knows how to play without him," Barcelona coach Pep Guardiola said. "We need to get a good result because it will be difficult if we have to win the series in San Mames."
Barcelona will look to maintain its excellent form against the Basque side. Guardiola's team has won 10 straight in all competitions, outscoring its opponents 42-4.
"We have to try to not be disheartened by Barcelona's numbers and how they are earning them," Bilbao coach Joaquin Caparros said. "We can all agree that we are seeing the best team in recent years or in history."
At Mestalla stadium, Valencia and Villarreal will deploy two of the best striker duos in Spain.
Villarreal's Nilmar and Giuseppe Rossi have combined for 19 goals in the league, while Valencia's Roberto Soldado and Aritz Aduriz have 11.
Both sides are having good seasons, with Villarreal in third place and Valencia in fourth. The two teams drew 1-1 in the league last month in a physical contest.
"We will go out to play a game with a lot of character with the attacking mindset that this team has," Valencia coach Unai Emery said. "It is a derby in a competition that is exciting for both teams, and the series will be a challenge."
On Wednesday, Real Madrid hosts Levante and Atletico Madrid plays Espanyol.
Levante held Madrid to a scoreless draw at home in September, but Jose Mourinho's team has won every match at Santiago Bernabeu stadium this season and will have every star player available except for the injured Gonzalo Higuain.
Levante will be hoping that striker Felipe Caicedo, who has scored in the last three games and has seven goals for the season, can play after he underwent medical tests on his ankle to miss Monday's training with his teammates.
Atletico Madrid will host Espanyol on Wednesday with a rested Diego Forlan returning to action after having sat out his team's 3-0 win over Malaga at the weekend.
Espanyol, for its part, will try to rebound from its 5-1 loss to Barcelona and ensure the series is still open for the second leg in El Prat-Cornella stadium, where the Catalan side has only been denied victory once this season.
"We want to bounce back in Vicente Calderon stadium and take a game that is going to be very complicated," Espanyol forward Luis Garcia said.
It could be Simao Sabrosa's last match for Atletico Madrid if club president Enrique Cerezo's words prove to be correct. The Portuguese winger has been linked with a move to Besiktas in Turkey.
Sevilla and Malaga will play the first part of an all Andalusian series. Both teams are struggling in the league - Sevilla has lost five straight games and Malaga has lost four of its past five.
Also on Wednesday, Almeria plays Mallorca and second-division leader Betis hosts a rejuvenated Getafe.
Cordoba, the only other lower-division club still in the competition, hosts Deportivo La Coruna on Tuesday.
"Just because they are a second-division team does not mean that we are going to take the game lightly," Deportivo coach Miguel Angel Lotina said. "Cordoba has eliminated a first-division club in Racing Santander. It is a hardworking team and it will be a difficult series."
