Argentina coach Gerardo Martino concedes he is worried about what the United States are capable of ahead of their Copa America Centenario semi-final meeting on Tuesday.
Hosts USA qualified for the semis of the tournament for just the second time following a hard-fought 2-1 win over Ecuador in the quarter-finals.
Clint Dempsey scored one and set up the other in the win, with both sides forced to play with 10 men for the second half after Jermaine Jones and Antonio Valencia were shown red cards.
Martino believes USA will provide a difficult test for last year's Copa America runners-up, especially if they show the form from their opening 45 minutes against Ecuador.
"I'm worried more about the United States from the first half in the game with Ecuador," the 53-year-old said ahead of the clash at NRG Stadium in Houston, Texas.
"I hope we can play a game that is more like the second half of that game."
Lionel Messi will start in back-to-back games for the first time this Copa after recovering from a back complaint.
The Barca star was instrumental in Argentina's 4-1 win over Venezuela in the last eight, scoring and assisting twice, and Martino said it is normal that USA will try to mark him out of the game.
"USA's football has grown a lot in recent years, but of course it is logical that they will try to neutralise the best footballer in the world," he added.