Japan produced the first upset of the Olympic Games when they beat potential gold medallists Spain 1-0 at Hampden Park, Glasgow on the opening day of the men's football competition.
Spain, bidding to add the Olympic title to their world and European crowns, finished with 10 men after midfielder Inigo Martinez was sent off just before halftime having found no answer to Yuki Otsu's 34th minute goal.
In the other early matches Morocco and Honduras drew 2-2 at Hampden, while Mexico and South Korea drew 0-0 at St James' Park.
There were five matches being played later today (NZ time) including favourites Brazil against Egypt and hosts Britain against Senegal.
Spain, among the favourites to repeat their 1992 Olympic success when they won gold as hosts in Barcelona, had never lost to the Japanese at any age-group level and the result was Japan's most impressive since they beat Brazil in the 1996 Atlanta Games.
The victory left Takashi Sekizuka's side top of Group D after Morocco and Honduras shared the points.
Spain fielded a strong team that included three players who featured in the squad that retained the European title earlier this month - Juan Mata, Jordi Alba and Javi Martinez.
While the Spaniards had plenty of possession, Japan held firm at the back and with sharper finishing could easily have won by a bigger margin especially in the later stages.
The breakthrough came when the Spanish defence were left in confusion at a corner, allowing Otsu, who plays for Borussia Moenchengladbach, to muscle his way into space and sidefoot home past Spain's Manchester United goalkeeper David de Gea.
Japan then benefited when Kensuke Nagai was brought down by Inigo Martinez just outside the penalty area and earned a straight red card from American referee Mark Geiger.
Morocco also finished with 10 men after defender Zakarya Bergdich was sent off 19 minutes from time against Honduras.