The Americans advanced through a group that contained Germany, Portugal and Ghana, which has set up a knockout round clash with the Belgians tomorrow.
When asked about his nation's opponent, Mertens told Sporza: "The US have surprised me. They are a real team and have a united group."
Meanwhile, the 27-year-old has expressed his determination to force his way into the starting lineup for the clash in Salvador.
"Hopefully I have proved that I can contribute, be that from the start or coming on as a substitute," the Napoli player added.
Belgium have won four of their previous five meetings with the USA.