Belgium and the USA will go head-to-head in Salvador on Tuesday, with a spot in the FIFA World Cup quarter-finals up for grabs.
USA joined Germany in progressing from Group G, after finishing second behind the Germans, ahead of more fancied opponents Portugal and Ghana.
The performances from Jurgen Klinsmann's men have come as a surprise to Mertens, with the Belgium winer bracing himself for a tough encounter at the Itaipava Arena Fonte.
''I think they (USA) did really well," Mertens told reporters on Sunday.
"I was a little bit surprised but I watched the games and I think they are a really good team.
"They have a good striker in front and the team behind is like a team.
"They are really good together so for us it's going to be difficult to score a goal but we are confident so we want to win this game."
Mertens and his teammates found it tough to break through solid defensive rearguards set up by Algeria, Russia and South Korea in Group H.
Belgium only managed to score four goals in three games - the equal-lowest amount by a group winner along with Costa Rica.
And the Napoli player is expecting the USA to adopt similar tactics on Tuesday.
''Playing against a team with ten players behind is difficult to score and that's what happened in the three (group) games," he said.
"I think this game is going to be the same.
''We know how they play, they play like a team. That's what they do the best.
"They have some players that can make the difference so it's going to be a hard game.''
Back-up goalkeeper Simon Mignolet is also aware Belgium must be at their best if they are to overcome the USA.
"You don't get out of a group with Germany, Portugal and Ghana to go into the second round if you aren't a good team," said Mignolet.
"I think United States will know that at the minute and it'll be a very difficult game for us on Tuesday.''
