The Atletico Madrid man saw the latter part of his season wrecked by fitness concerns as he limped out of his side's La Liga decider at Barcelona and then the UEFA Champions League final against Real Madrid without contributing significantly.
He then faced a race against time to prove his fitness to Vicente del Bosque, but did enough to be in Spain's 23-man squad when it was announced on Saturday at the expense of Fernando Llorente and Alvaro Negredo.
Spain's final warm-up game sees them tackle El Salvador in Maryland, USA next Saturday, having beaten Bolivia 2-0 in Seville on Friday in Costa's absence.
After scoring 36 goals in 51 games for Atletico last season, Costa is expected to be a crucial part of his adopted nation's plans and the 25-year-old is raring to go.
He told Marca: "I was very excited about the prospect of representing Spain at the World Cup in Brazil. I'm delighted.
"I'm doing great. I've been working with the physios and fitness coaches over the last few days and I feel really good.
"From (Sunday) I'll start working with the rest of the team under Vicente del Bosque's orders.
"I'm hoping to get a run-out in next week's game against El Salvador in Washington."
