The Belgium attacker admits he has been taken aback by the form of his side's Round of 16 opponents at the World Cup so far
Jurgen Klinsmann's side finished second in Group G at the expense of Portugal and Ghana to book a knockout tie with the Red Devils, and Napoli star Mertens says they have really impressed him so far.
"I think they did really well, I was a little surprised," Mertens told reporters at Belgium's Mogi das Cruzes training base. "I watched their games and they have a really good team. They have a great striker in front and the team behind is like a team, they are really good together.
"So for us it is going to be really difficult to score a goal, but we are confident and want to win this game also."
While Belgium won all three of their fixtures in Group H, Marc Wilmots' men have managed only four goals so far against Russia, South Korea and Algeria.
Mertens, however, is sure that things will click in front of goal for them soon.
"We know against a team that plays with 10 players behind the ball it will be difficult to score, and that's how we played the three games. I think this game is going to be the same," he said.
"I think the most important thing is to win this game, because if we don't win it we are out."
Reserve goalkeeper Simon Mignolet insists that the Belgians will not underestimate the opposition when they meet in Salvador on Tuesday.
"I know everybody's saying we are the favourites but everybody knows how good the United States are, and you don't get out of a group with Germany, Portugal and Ghana to go into the second round if you're not a good team," Mignolet stressed.
"The United States are here on merit, and it will be a very difficult game for us on Tuesday. Obviously we have already competed with the United States twice in recent years, but that doesn't mean it's going to be an easy one."