Spain coach Luis Enrique blamed five minutes of "total loss of control" for his team's shock 2-1 defeat by Japan on Thursday, which saw them qualify for the World Cup last 16 second from Group E.
And Japan defender Yuto Nagatomo said the Blue Samurai showed they have the "guts" to compete at the World Cup after stunning La Roja to advance to the last 16.
Japan upset both Germany and Spain to finish top of Group E, digging deep to come from behind to grab gritty victories in both games.
Hajime Moriyasu's side will now face Croatia in the last 16 and Nagatomo believes they can pull off another shock against the 2018 World Cup finalists.
"No one in the world believed that Japan would finish top of this group – no one," said Nagatomo.
"But we believed, and we believed during the game today. We showed the world what Japanese guts looks like."
La Roja, among the favourites to win the tournament, would have been eliminated if Costa Rica had held on to beat Germany, although Los Ticos eventually fell to a 4-2 defeat.
Alvaro Morata sent Spain in front early on but two strikes in four minutes from Japan at the start of the second half turned the game on its head.
"[It was] five minutes of total lack of control, of panic, even," said Luis Enrique.
"They scored two goals against us, and if they had needed to score two more, then they would have scored them too, I'm sure.
"This is a World Cup and when a team like Japan does not have anything to lose, well, they decide to risk it all and flew past us like they were aeroplanes.
"The situation was under control once Japan decided to recover their previous position on the pitch and shut up shop at the back."
Ritsu Doan netted the equaliser for Japan in the 48th minute before Ao Tanaka bundled home an extremely controversial second.
The ball appeared to go out of play before Kaoru Mitoma crossed the ball to him, but after a long VAR check, officials granted the goal.
"I have seen a photo that must have been tampered with. It cannot be that this photo is real. It has to be manipulated," said Luis Enrique.
"I felt that something fishy was going on when the VAR took as much time as it did to decide ... I have nothing to say.
"Luckily the team only goes into collapse mode once every four years, otherwise I wouldn't be able to cope."
Mourning and warning
Spain will face Morocco in the last 16 after the defeat and Luis Enrique believes his team did not deserve to finish as group winners.
"Japan go through first and it's deserved because they beat Germany and Spain," said the coach.
"The mourning and the pain, we'll go through that tonight, there's nothing to celebrate.
"We have gone through second, yes, but this has to serve as a warning to be aware that this is going to happen.
"If against Morocco we have the fortune to go in front, Morocco, will risk it all at full pelt again.
"We have to show more strength in the moments when the opposition is pressing us."
Luis Enrique was on the edge of his technical area during the game and did not realise there was a period of time where Spain were heading for elimination.
With Costa Rica 2-1 up against Germany, Spain were temporarily in third place for a few minutes, before Kai Havertz levelled the game at 2-2.
"We were going out?" asked Luis Enrique. "I did not know it because I was focused on my game, not speculating [on the other]. If I had found out, I would have had a heart attack."