Costa Rica left it late but managed to stun Japan as Keysher Fuller scored the only goal to win the Group E clash at the Ahmad bin Ali Stadium.
Defender Fuller fired a late winner with Costa Rica's first shot on target in the 81st minute to throw the group wide open and bring Japan crashing back down to earth after their stunning opening win against Germany.
Japan boss Hajime Moriyasu made five changes and was punished for resting key players as his side were unable to replicate those heroics.
Costa Rica had been thrashed 7-0 in their opening game against Spain and, after frustrating Japan for large periods, they snatched an unlikely victory.
Head coach Luis Fernando Suarez responded to the Central Americans' capitulation against Spain by making just two changes and his side was content to limit Japan to few chances.
Japan were big favourites going into the match and they were first to threaten through Ritsu Doan's shot from outside the box.
Costa Rica's first attempt came in the 35th minute from Francisco Calvo, who was also off target with his long-range effort.
Neither side created another chance of note in a disappointing first period, but Costa Rica's Paris St German goalkeeper Keylor Navas saved well from Hidemasa Morita's shot soon after the restart.
Japan sent on Takuma Asano and Hiroki Ito at half-time and they added an intensity that had been sorely lacking.
Navas was forced into action again to keep out Asano's header, while Ko Itakura and Wataru Endo had successive shots blocked.
Costa Rica offered next to nothing in attack, but stayed in the game as further efforts from Japanese pair Daichi Kamada and Yuki Soma were also blocked.
Japan were then caught cold with nine minutes remaining as Costa Rica made a rare foray forward.
Yeltsin Tejeda pounced on Hidemasa Morita's mis-placed pass and slipped in Fuller on the edge of the box and he curled a left-footed finish into the top corner, goalkeeper Shuichi Gonda getting his hands to the shot but not able to keep it out.
Japan responded with their best chance of the match in the 88th minute, but Kamada's effort from inside the area was saved by the feet of Navas.
