Leading into the tournament, the 22-year-old had endured difficult times in the Albiceleste shirt under coach Diego Maradona. He still averages less than a goal every three games at international level, a disappointing tally for the reigning FIFA World Player of the Year.
However, he feels a different man playing for Argentina this summer in South Africa. "Ever since we arrived here, my mentality has changed," said Messi. "I went through difficult times with the national team when things didn't come out the way I wanted to.
"I was not only me but also my team-mates. The qualifying round was not good and it wasn't pleasant to go through that but I've stood up and it was great. I really enjoyed the game against Nigeria."
Argentina edged Nigeria 1-0 in their Group B opener, courtesy of a Gabriel Heinze goal. Messi enjoyed plenty of chances but was denied by Nigeria goalkeeper Vincent Enyeama, who secured the Budweiser Man of the Match award following a string of superb saves.
"We played a good first game and we created a lot of chances," said Messi. "We were able to get clean balls up front and that is what you need. If we play as we did against Nigeria, we can be calm."
Messi feels Maradona is having a positive effect on the team. "Maradona is just another of the group," he said. "He gets closer to players. He was a player and knows what happens to us. He gives us a lot of confidence."
Argentina fans are hopeful Messi will continue to show the form he had for Barca this season. He won the European Golden Boot after netting 34 goals and also finished as the leading scorer in the UEFA Champions League with eight.
Messi, however, has reiterated that Argentina are not a one-man team. "This national team does not depend on me," he said. "We have many players that can determine a game. I depend on the midfielders to bring the ball up in order to create chances."
Argentina take on Group B rivals Korea Republic on Thursday looking for their second straight win in South Africa.