Belgium suffered an injury scare when defender Glen de Boeck was taken to hospital for x-rays on a knee injury sustained in the 1-0 win against Costa Rica.
De Boeck was expected to start with Celtic's Joos Valgaeren and Philippe Clement of Club Bruges ruled out with injury.
But he now faces an anxious wait to discover if he has to pull out of the squad ahead of the World Cup kick-off against co-hosts Japan on 4 June.
Belgium made the most of few chances to beat Costa Rica courtesy of Bart Goor's lone strike.
Costa Rica dominated for long periods, but Paolo Wanchope and his colleagues were denied by an outstanding performance from Belgium goalkeeper Geert De Vlieger.
Belgium took the lead in the 23rd minute when Marc Wilmots floated a free kick into the penalty area, forward Gert Verheyen headed back to Goor and the Hertha Berlin midfielder cracked a left-footer which gave goalkeeper Erick Lonnis no chance.
The game at KKWing stadium in Kumamoto was played in searing heat in the mid-80s in front of a 20,000 crowd.
"We had several opponents" said Belgium coach Robert Waseige. "Not only an excellent Costa Rica but also the heat."
Costa Rica open up agains China on 4 June before facing Brazil and Turkey in Group C.
Wanchope was in optimistic mood, despite the defeat.
He said: "The best thing to remember is that we have the ability to create chances. During the World Cup we just have to concentrate better."
Costa Rica should have gone ahead in the fourth minute when Wilmer Ropez found Wanchope totally unmarked on the penalty spot.
But even with goalie De Vlieger totally out of position, the Manchester City forward inexplicably failed to tap the ball into the empty net.
Five minutes later he volleyed just wide from close range.
In the 15th minute, Ronald Gomez broke free and forced a great save from De Vlieger with a long-range right-foot shot and Carlos Castro did likewise a minute later, underlining Costa Rica's early dominance.
Costa Rica came out strong after the break and substitute Rolando Fonseca thought he had scored in the 51st minute but the goal was disallowed for offside.
Three minutes later, a bad backpass from Sven Vermant put Wanchope clear again, but the forward squandered this chance too, lifting the ball over goal.
In the dying minutes, De Vlieger pulled off another great save from substitute Steve Bryce.
Costa Rica: Lonnis, Castro (80 Vallejos), Wright, Martinez, Wallace (76 Steve Bryce), Centeno, Marin (88, Juan Rodriguez), Solis, Ropez (46 Fonseca), Wanchope (73 Parks) , Gomez (76 Medford).