front of a partisan Brazilian crowd of 73,000 at Rio de Janeiro's Maracana stadium.
The loss left Spain without the only major title it has failed to win in a five-year span in which it has dominated the game.
"Once in a while it is convenient to lose because we don't think we are unbeatable," Del Bosque said. "Today they had more energy and certainly kept their shape better than we did."
The World Cup winners were beaten in every phase of the game, a fact Del Bosque conceded. Brazil scored early and late in the first half - Fred and Neymar doing the damage - with Fred scoring early in the second to seal the victory.
The last goal prompted jubilant Brazil fans to begin chanting: "You want to play, you want to play. Brazil will teach you how."
In the 68th when defender Gerard Pique was sent off on a straight red card, the crowd began chanting the name of his girlfriend - pop singer Shakira - who was sitting in the stands.
It was that kind of night for Spain, which started poorly and never recovered after Fred's goal in the second minute.
"The first minutes and the last minutes of the halves are critical," Spain defender Cesar Azpilicueta said "And they scored their three goals at the beginning and ends of the halves, which is the worst time. Those are the most demoralizing moments."
The victory ended Spain's unbeaten string of 26 games, but the Spanish coach played down any need to re-build his team with next year's World Cup in Brazil just a year away. Spain has yet to qualify, but is almost certain to.
"We are happy with what we have done over the last few years," Del Bosque said. "But one loss - you have to look at it, but not overreact to it. We are not content with the loss. But when a team is superior, you have to accept it. It was a deserved defeat."
Midfielders Andreas Iniesta and Xavi Hernandez were ineffective in the match, usually beaten to loose balls by the quicker Brazilian, who were playing with an extra day of rest. Spain looked jaded after a tough extra-time victory on Thursday over Italy, 7-6 on penalties.
"We knew we were going to encounter a physical game with lots of fouling," Iniesta said. "We lost to a very strong team and the small details lets us down."
As the match began, hundreds of police and soldiers barely 100 yards (meters) from the stadium let off volleys of tear gas, attempting to disperse protesters as they marched on the venue.
Protesters have demonstrated at almost every match in the two-week Confederations Cup, complaining about $13 billion being spent on next year's World Cup in country that has notoriously poor hospitals, schools and social services.
Del Bosque was asked about the violence outside the stadium.
"This is the first I've heard of it," he said. "We have been treated well here and impressed by our hosts."