Spain seek showdown with Brazil
"The team I haven't had the chance to meet and would like to face are Brazil," the goalkeeper told a news conference ahead of Tuesday's friendly against Argentina at River Plate's Monumental stadium.
"We would love to play a match with Brazil," said Casillas, his face lighting up. "I think the last (meeting of the sides) dates back 12 years."
Spain met twice world champions Argentina, Brazil's arch rivals, as recently as last November in a friendly the home team won 2-1 in Madrid.
Casillas said his team could not afford to rest on their laurels as they faced a 2012 qualifying campaign where they are defending their European championship title.
He said friendlies like Tuesday's against Argentina were important for Spain to keep progressing, saying: "It's the first time we've travelled so much for a friendly but it's well worth it".
Spain played their opening Euro qualifier in Liechtenstein on Friday, recording a 4-0 win, and flew about 15 hours to Buenos Aires on Saturday.
Coach Vicente del Bosque said: "The worst that could happen to us is to get comfortable and think ourselves kings of the world.
"Now we've finished with the objective of winning a World Cup and we're preparing to validate our European title."
Del Bosque said Spain were not favourites on Tuesday.
"I think Argentina are in a position to be able to beat us," said Del Bosque, whose side scored a stoppage-time equaliser to draw 1-1 in Mexico last month in their first match since lifting the World Cup.
"They have an organised game and they have talent, if the two things work they will be on a level with our team. We're not so far apart."
Del Bosque said Argentina had moved on from the World Cup in South Africa where former coach Diego Maradona's team were beaten 4-0 by Germany in the quarter-finals in July.
Tacitly praising new coach Sergio Batista, whose first match was a 1-0 win in Dublin last month, he said: "From what I saw against Ireland they have that pause and possession in midfield and change of acceleration through the forwards to give their attacking game movement."