The Atletico Madrid forward was a major injury concern for coach Vicente Del Bosque after picking up a hamstring injury that saw him limp out of the Champions League final.
But the Brazil-born international is confident his injury problems are behind him and that he will be ready to play a big part in Spain's quest to retain the World Cup.
"I'm very happy," he told AS. "I am buzzing to with Spain to play in the World Cup in Brazil. I'm delighted.
"I was working these days with the physios and rehabilitation coaches and I feel great. Starting tomorrow, I will start and work with the team under the command of Vicente del Bosque. I hope to have a few minutes in the game next week against El Salvador in Washington.
"We will fight for the World Cup."
Last month the striker's Atleti side was just seconds away from a first ever Champions League crown before Sergio Ramos's last-minute header forced the tie into extra time.
Real Madrid went on to beat Atletico 4-1 and Diego Costa says Carlo Ancelotti's men were lucky to win La Decima.
"Real Madrid were lucky," he laughed.