Costa was given a warm reception by the handful of fans who turned out to watch the world and European champions' training session on a chilly night at the national team's headquarters in Las Rozas near Madrid.
Coach Vicente del Bosque called him up for Spain's two friendlies in November but he had to withdraw due to injury and the in-form 25-year-old appears well placed to earn a place at the World Cup finals in Brazil ahead of the likes of Fernando Torres and David Villa.
He has scored 21 goals for Atletico Madrid in La Liga this season, second only to Real Madrid's Portugal forward Cristiano Ronaldo on 23, and has struck five times in the Champions League to help Atletico to the brink of the quarter-finals.
"I think he is a great choice and he can bring a lot to the team," Spain and Real Madrid centre back Sergio Ramos told reporters on Monday as he arrived at the team's training camp.
He dismissed suggestions there was bad blood between him and Costa, who has clashed with Ramos in recent Madrid derbies.
"There is no problem, that is just part of our sport," Ramos said. "Football is for smart people, we are team-mates in the national team now and a different way of behaving is required."
Costa's commitment to Spain angered Del Bosque's Brazil counterpart Luiz Felipe Scolari, who withdrew an invitation to play in friendlies against Honduras and Chile and accused him of "turning his back on the dream of millions".
He represented Brazil in friendlies against Russia and Italy last March but was left out of the squad for June's Confederations Cup and is not tied to one country until he has played in an official competitive game.
Spain's Barcelona midfielders Xavi and Cesc Fabregas and Ramos did not take part in Monday's training session and instead worked in the gym, the Spanish federation (RFEF) said on the national team's website (www.sefutbol.com).
Wednesday's game at Atletico's Calderon stadium is a repeat of the Euro 2012 final, which Spain won 4-0, and they also beat Italy on penalties in the semi-finals of last year's Confederations Cup in Brazil.