Del Bosque: USA caught Spain by surprise
"They had huge energy, were very quick in attack and caught us by surprise," he told reporters after the Americans set up a final clash with Brazil or hosts South Africa on Sunday.
Goals from Jozy Altidore and Clint Dempsey ended Spain's world record run of 15 successive victories and their 35-match unbeaten sequence, a world record streak they share with Brazil.
"It's never good to lose, we are not used to losing any more," added Del Bosque.
"Every defeat hurts but we will now try to win our next game (Sunday's third place playoff) and then turn our attention to securing World Cup qualification."
Del Bosque was pleased with Spain's workrate and said all they lacked was decent finishing.
"We cannot be unhappy with our effort," he said. "We tried everything to win.
"Today we missed that final touch. We had many opportunities but they really closed the door for us.
"We have to accept this defeat but I believe we are still formulating a team that can go on strongly. We are at peace, even in defeat."