Porto goalkeeper Iker Casillas has likened his relationship with Jose Mourinho at Real Madrid as a "love" affair that disintegrated.
Casillas and Mourinho fell out early on during the coach's three-year spell in charge of Madrid, and the keeper found himself sidelined in the final months of the 2012-13 season.
Casillas, who was restored as Carlo Ancelotti's first choice at Madrid in the 2013-14 campaign, has spoken about his turbulent past with Mourinho in an interview with Movistar+ show "Fiebremaldini."
"At the beginning there was a lot of connection, we interacted a lot, but in the end, there wasn't a very good feeling," he said. "It's like a couple, at the beginning you are very much in love and then it is no longer enough."
Madrid, who last won La Liga under Mourinho in the 2011-12 campaign, will take a big step towards lifting the domestic title if they beat Barcelona at the Santiago Bernabeu on Sunday in the Clasico.
With six games of the season remaining, Zinedine Zidane's Madrid side are three points clear of Barcelona at the top of the standings and have a game in hand over their rivals.
Casillas spent 25 years at Madrid and made 725 appearances for the club before announcing he would join Porto in the summer of 2015.
Casillas, who played in his first Clasico in February of 2000, knows the importance of Sunday's encounter for his former club.
"That rivalry exists and I have experienced it," he said. "In my experience, in a given game, we hurt Barcelona much more. My hope is that Madrid win and they can claim the league title because in my opinion, they have deserved it."
Casillas, who has kept 17 clean sheets in 29 Primeira Liga starts for Porto this season, has not given up hope of returning to the Spain national team. Under his captaincy, Spain won their first World Cup in 2010 and retained the European Championship two years later.
Casillas has not been called up to the national team since Euro 2016, where he was the second-choice keeper to Manchester United's David De Gea.
"I have not given up on the national team and I'm not going to," he said. "The day that I do, I will be the one to announce it. Right now I have a challenge against myself. I would like to go back but it's the coach's [Julen Lopetegui] decision."