What you need to know:
- Substitute Junya Ito looked to have wriggled free and was closing on goal, only for Costa Rica defender Francisco Calvo to cynically tug him back on the edge of the box, earning a yellow card that could easily have been red.
- Costa Rica had done nothing for most of the game as an attacking force, but nine minutes from normal time they stole it when Fuller's shot took a deflection and looped in.
Costa Rica stunned Japan 1-0 thanks to a late goal at the World Cup on Sunday to blow Group E wide open and hand a potential lifeline to Germany.
Japan, who shocked four-time champions Germany 2-1 in their opener in Qatar, could have taken a giant stride towards the last 16 with a win.
Instead they laboured for large parts of the match against a Costa Rica side who were hammered 7-0 by Spain, then defender Keysher Fuller scored in the 81st minute with a deflected strike.
It was Costa Rica's first shot on target at this World Cup and could be of huge benefit to the Germans.
The result left Spain, Japan and Costa Rica all on three points. Spain face Germany later Sunday in another crucial contest.
Japan made five changes to the team which shocked Germany in their opening match as coach Hajime Moriyasu made full use of his 26-man squad.
In temperatures of 32 degrees Celsius (90 degrees Fahrenheit) at Ahmad Bin Ali Stadium, Japan immediately went on the attack and won a corner within 30 seconds to signal their intent.
Costa Rica coach Luis Fernando Suarez had resisted making wholesale changes to the team thumped by Spain, dropping teenage winger Jewison Bennette and defender Carlos Martinez to the bench.
Gerson Torres and 34-year-old centre-back Kendall Waston took their place in an ageing XI which featured four players from the 2014 World Cup quarter-finals.
But Japan's early promise melted in the Doha sun and the first shot at goal -- a tame effort which sailed high and wide -- did not come until 10 minutes from half-time through Costa Rica's 30-year-old Joel Campbell.
A dreary first half came to a close with "Los Ticos" having had more of the ball but neither side mustering a shot on target.
Moriyasu had seen enough and made a double change at the break, one of them the introduction of Bundesliga-based striker Takuma Asano, who scored the winner against the Germans.
The Blue Samurai were immediately more cutting and midfielder Hidemasa Morita drew the first proper save of the match within seconds of the restart with a powerful strike.
Moriyasu threw on Brighton attacker Kaoru Mitoma for a defender just after the hour as Japan chased the victory that would put them on the cusp of the last 16.
It was all Japan as the match ticked into the last 20 minutes in front of 41,000 spectators.
Substitute Junya Ito looked to have wriggled free and was closing on goal, only for Costa Rica defender Francisco Calvo to cynically tug him back on the edge of the box, earning a yellow card that could easily have been red.
Costa Rica had done nothing for most of the game as an attacking force, but nine minutes from normal time they stole it when Fuller's shot took a deflection and looped in.