The former Real Madrid boss, who has helped steer Spain to a record-breaking 15-match win streak, said he respected Bob Bradley's team who have upset the odds by making it to the last four at the expense of Italy.
"We have a less strong rival in front of us (than Italy) but we must tackle it in a sporting spirit," he said ahead of Wednesday's match in Bloemfontein, where the temperatures took a tumble on Tuesday. I don't think we are that much better than them. They are physically and tactically strong and they can make life difficult for us."
Regardless of what Del Bosque says, Spain are overwhelming favourites to make the final on Sunday against either Brazil or hosts South Africa. They won all three of their group games, scoring eight goals and conceding none to match Brazil's record 35-game unbeaten run.
Del Bosque was not prepared to look beyond their semi-final when asked to comment on playing Brazil for the trophy. "We have a clear rule - we believe in going from one match to the next," he said.
"It's been mentioned here that we had a weak group but it took us a lot of work to win. We must always be aware that we have a rival that can create fatal difficulties for us. We can be happy with what we have done in the group stage but it will be meaningless if we don't take the title. Of course all our victories will go down in history, but we have to look forward and not backwards."
The 58-year-old has been busy analysing videos of the United States and pinpointed LA Galaxy star Landon Donovan and Fulham's Clint Dempsey as capable of causing his team problems. "In midfield and in attack they have strong players," he said.
"They like to attack. Donovan is very good and Dempsey is strong at headers and in set pieces. On Sunday against Egypt they played with a central midfielder who we did not know (Charlie Davies) who played well so we must really watch out for this opponent."
Barcelona midfielder Xavi, widely seen as the star of the tournament so far, warned that the pressure would be on Spain as favourites, with most people expecting them to coast through. "We accept the role of being favourites but perhaps there is more pressure on Spain than the Americans because of this," he said.
"I've seen the United States and they are rather strong. Their football has been evolving, they've improved their attacking skills, they have talented players and they will not be an easy opponent. We may be favourites but it won't be an easy match."