Midfielder Xabi Alonso believes Spain will have to be at their perfect best to retain their European Championship title this summer.
Spain have been the dominant force of world football since winning Euro 2008, highlighted by winning the 2010 World Cup and embarking on a record-equalling 35-game unbeaten run.
Vicente del Bosque's side have, however, shown some signs of vulnerability in the past 12 months losing friendlies in England and Italy.
"It is increasingly more difficult to keep winning," Real Madrid midfielder Alonso told GQ in an interview to be published on Thursday.
"We have a good team, we have the experience, but there are also other teams that have prepared well and are very eager.
"I think it will be even more difficult than our World Cup win because we are going to have to do things almost perfectly to win it."
There are fears in Spain that a chaotic domestic season, when Madrid ended Barcelona's own recent dominance against a backdrop of ill will, could affect morale in the national team.
Alonso won his first Primera Division title but denied Barca's players would be jealous, claiming instead they would be driven to help Spain and prove claims of a Catalan division are incorrect.
"The Spanish team is independent of that; it has nothing to do with it," he said.
"We have always differentiated between the two. There has been much hype and talk on the subject and after a while you tire of it all. You get asked, you answer and then one thing or another is written.
"In the end the players do not ask, write and go. It has nothing to do with the way the players work together when we are with the Spain squad and fighting for. We fight for it no matter what happens at the end of the season.
"If someone has won something, or if you've won, then you congratulate and know that you are now fighting for something else."
Spain begin Euro 2012 against Italy in Gdansk on June 10, with group games against Republic of Ireland and Croatia to follow.