Wataru Endo marked his return to the scoresheet for Japan after a three-year hiatus. However, his lone goal proved insufficient as Japan faced an unexpected setback against Iraq.
Ayman Hussein's first-half brace for the underdogs set the tone for an unlikely triumph, leaving Japan scrambling to recover.
Despite Endo's late 93rd-minute goal, courtesy of Liverpool's midfielder, it came too late to salvage the match for the four-time tournament winners, putting them at risk of an early exit.
Although Japan, the highest-ranked nation in Asia, secured an initial victory against Vietnam, the unforeseen defeat now forces them to secure a positive result in their upcoming group-stage encounter.
Notably, Iraq, positioned 63rd in the world rankings, outperformed expectations against the formidable Japanese squad.