Messi has had an outstanding year with the Catalan club, helping to win the Spanish championship, the Champions League, Copa del Rey and Club World championship.
But he has struggled for Diego Maradona's Argentina, who crawled into the World Cup with a 1-0 win away to Uruguay in the last of their 18-match campaign and face Nigeria, South Korea and Greece in their first round group in South Africa.
"I don't know," Messi told reporters before FIFA's World Player of the Year award ceremony.
"I try to do the same as I do for Barcelona. But in the World Cup the qualifiers are complicated, sometimes it's difficult to play and it was a big effort for us to qualify.
"I believe the World Cup could be completely different for us, I believe it could be better for me and Argentina."
"It bothers me a lot," he said. "I'm Argentine and I have a feeling for the shirt. I give everything for the Argentina team."
The former Manchester United player said he believed they could spring an upset and drew inspiration from Greece's 2004 European championship win, when they beat Ronaldo's Portugal in the final.
"I have never won anything important for the national team but it's important for me," he said.
"Portugal have a chance, nobody spoke about Greece in 2004 and they won the tournament. I'm looking forward to a great World Cup, I'm really focused and happy to be there."
