The late victory against a well-organised Morocco via Kensuke Nagai's long strike sent Japan into the quarter-finals for the first time since the 2000 Sydney Olympics
The 28-year-old FC Tokyo player, who also represented Japan in the 2004 Athens Games, was chosen along with Maya Yoshida as one of the team's two over-age selections. The two veterans kept Japan's back line steady against a Morocco squad that was far more up to the task than Spain just three days earlier.
"It's a relief to escape the group stage with one match remaining," Tokunaga said after the match. "We held together and defended, and to take three points from a match like this shows how we've grown.
"Now that we've accomplished one goal, we want to play for a medal."
Yoshida, also playing in his second Games following Beijing 2008, believed that Japan did themselves a disservice by wasting chances.
"Things would have been a lot easier if we'd converted the chances we had," the VVV defender told reporters. "I'm disappointed we couldn't convert our set pieces in either half.
"We were still fatigued from the Spain match, but we didn't give up a goal in the first half and were able to find our momentum in the second."
Coach Takashi Sekizuka's side was praised by none other than Samurai Blue manager Alberto Zaccheroni, who watched from the stands.
"It was a fantastic result," the Italian commented. "I thought it would be a tough group, so the accomplishment is twice as impressive. Everyone's worked hard."