The Spanish keeper faced a vast array of world-class strikers during his time at the Bernabeu, but said it was the Argentine - football's most formidable striker - that he found hardest to play against.
Casillas played and often conceded against Messi in a number of El Clasico fixtures before ending his Los Blancos career this summer to join Portuguese outfit FC Porto.
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"It was always Messi. I’ve played him on a number of occasions", he said, as per Goal.com reports.
He maintained, though, that the rivalry he shared with Messi on the pitch was one both he and the prolific forward thoroughly enjoyed.
The 34-year-old came up through the Galacticos youth system and spent 16 years at the club, playing 725 games in all competitions before bidding an emotional farewell to Madrid and moving to the Estadio do Dragao.
Speaking about the best saves he made during the entirety of his career, Casillas said: "The saves I made against Bayer Leverkusen (in the 2002 Champions League final) and the one in the World Cup final (in 2010) to deny Arjen Robben [are my favourites].”
He believes that mental strength remains the key attribute to succeed as a goalkeeper.
“You need to be mentally very strong. Being a goalkeeper involves a lot of risk, courage and intelligence. I enjoyed being a goalkeeper since I was young.”
The Madrid legend has been excellent for Porto this season having featured in six league matches, keeping four clean sheets in the process.