The 25-year-old forward had a fine season with Atletico Madrid in 2013-14, scoring 27 goals in 35 league games as the Rojiblancos won La Liga, although he then limped out of his side's 4-1 Champions League final defeat to Real Madrid after just nine minutes.
However, the striker insists he has fully recovered from the hamstring problems that affected his final weeks of the campaign and says he is ready to perform at his full potential for La Roja.
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"I'm feeling very good," he said during a news conference Tuesday. "At this moment, I think I'm feeling the best I ever have — perfect. I feel I'll have no problems at the World Cup.
"I always have anxiety, even more so before playing against a team like Holland. But players try to just live in the moment and I hope to get going as soon as possible. As I said before, everyone is treating me in a special way and I'm enjoying a beautiful moment. I can only give my thanks."
The Brazil-born attacker courted controversy in October after stating his intention to play for Spain rather than his native Brazil, prompting Selecao boss Luiz Felipe Scolari to accuse him of "turning his back" on his country.
However, Costa feels that he has nothing to be ashamed of and says even his family has no qualms about seeing him represent a rival nation at this summer's tournament.
"My parents are very happy to be close to me," he said. "They know why I chose Spain and they have always supported me."
Spain and the Netherlands face each other in Salvador on Friday in a repeat of the 2010 World Cup final, which saw Vicente Del Bosque's men crowned champions after a 1-0 win.