He thought Americans were a race of mongrels who could not stand up to his Ayran Supermen. Here's one of his quotes.
don't see much future for the Americans... It's a decayed country. And they have their racial problem, and the problem of social inequalities... My feelings against Americanism are feelings of hatred and deep repugnance... Everything about the behavior of American society reveals that it's half Judaized, and the other half negrified. How can one expect a State like that to hold together?
Well Hitler did have a screw loose, and became more loose with the passage of time. And he did look down on Americans as a “degenerate” nation, for all his various bizarre notions of race, etc...
He also discounted the extent of our industrial capacity, and ability to mobilize and project that power into Europe.
But in the end, I think his final calculations were skewed by being a victim of his own success. When he declared war on the United States, he had quickly conquered Poland, Scandinavia, the Low Countries and France. Britain was expelled from the Continent and isolated. The Wehrmacht was in sight of the Kremlin (the Soviet Counter-Offensive, from which the Wehrmacht would never fully recover, had only just begun).
Hitler thought at the time he was unbeatable.