"It is not my best Copa America, I am not having the one I had hoped," the Barcelona star said after Argentina's 2-0 quarterfinal win over Venezuela, ESPN reported.
"The important thing is that we won."
Striker Lautaro Martinez scored for the second game running to put Argentina in front in the 10th minute with a clever back-heeled finish, while substitute Giovani Lo Celso scored Argentina's badly-needed second goal in the 74th minute on a loose ball that squirmed through the hands of Venezuela goalkeeper Wuilker Farinez.
Messi has only scored once at the tournament in which Argentina has struggled offensively. The Argentines scraped into the last eight with a nervy 2-0 win over Qatar in their final group-stage game after losing to Colombia and drawing with Paraguay.
The six-time Pichichi Trophy winner added that he knew that Venezuela "was going to shut it down and look for the counter."
"We controlled the game from the beginning, the first goal came and from there we became ourselves," the man from Rosario said.
"In the second half, we knew they were going to move forward and leave spaces. We were firm throughout the game, it was a complete game."
Argentina coach Lionel Scaloni did not want to be drawn on Messi's poor display in the postgame news conference but said his presence alone is good for the team.
"Messi's contribution on the pitch is essential, if you could see what he gives us in the dressing room you'd think differently. I assure you it's great to have him here," Scaloni said.
When further pressed on Messi's poor form, Scaloni replied curtly, "All there is to say about Leo is he's the best in the world."
Friday's win at the Maracana marked the first time that Messi had played at the famed Rio de Janeiro stadium since the 2014 World Cup final loss to Germany. He blasted the condition of the field on Friday, calling it an "embarrassment."
"It doesn't help to play the ball fast and you always need some time because it doesn't roll well," Messi said.
"In all the pitches, the ball appears to be like a rabbit, it bounces all over the place. It is how it is and we have to adapt."
With the victory of La Vinotinto, Argentina will now face eternal rival Brazil for the first time in the tournament since the 2007 final, which it lost 3-0.
"We are happy to get to the semifinals. We have Brazil and it will be a tough opponent because they have great players. At a collective and individual level, they're strong plus they are the host. Today I believe we gave another step forward."