Last year's 3-0 defeat to Brazil in the final of the FIFA Confederations Cup has resulted in Spain being questioned in some quarters ahead of the World Cup, despite the fact they have won three major tournaments in a row.
And back-up keeper Reina believes it is getting tougher for Vicente del Bosque's side to maintain their dominance of the international scene.
"We're going to try to become world champions again but we know that it's going to be difficult," Reina was quoted as saying by AS.
"Other teams are catching up to us and it just keeps getting more difficult."
Reina has been part of Spain's all-conquering squads at Euro 2008 and 2012, as well as the 2010 World Cup.
And the 31-year-old, who spent last season on loan at Napoli from Liverpool, said playing a World Cup in Brazil made it more special.
"This World Cup is a bit of an obsession for me because it is going to be really special," Reina said.
"We'll be playing in the cradle of football."
