Argentina set up a blockbuster semi-final against host Brazil by beating Venezuela 2-0 at the Maracana on Saturday (AEST).
Messi has scored just once in Argentina's four games, and his goal came from the penalty spot against Paraguay in the group stage.
The Barcelona star, whose team faces Brazil on Wednesday (AEST), accepted he was yet to hit top form at the tournament.
"The truth is that it's not been my best Copa America, not what I expected," Messi said.
"There are matches that occur in this way. You cannot play a lot, it's complicated for those of us who want to do something different, to avoid rivals.
"The important thing is that we won and we continue."
Messi started in behind the front two of Lautaro Martinez and Sergio Aguero against Venezuela, with the Inter Milan forward getting on the scoresheet for the second straight game.
After the win, Messi was happy with the attacking trio, while he slammed the pitches, which have been criticised throughout the Copa America.
"We know it is fundamental that if we want to play the top three, we must run and help in defence," he said.
"It is a cup match, anyone beats anyone. It is difficult to play, the pitches are bad. It is a shame that all the pitches are like this in a Copa America."