Whichever team wins will advance to the quarter-finals of football's showpiece tournament for the first time in their history.
Japan have only once made it past the group phase when they progressed to the second round of the 2002 tournament on home soil.
But while reaching the quarter-finals would be an amazing achievement, coach Takeshi Okada said it is just a stepping stone on the way to bigger and better things.
"I would say for the football community it will be wonderful and encouraging news," he said.
"What the Japanese football community is aiming at, which is to become one of the top teams, will be one step closer if we qualify for the final eight.
"But I have to say if you only pile bricks vertically they will eventually fall down, so we have to support from both sides.
"I personally don't want to base everything on just one result or just one competition. We need to have a longer-term vision, to estimate and evaluate the level of Japanese football."
"We need to be aware of the journey and the path which is still ongoing.
"This World Cup, for the fact we've been able to get this far, it is not only a credit for myself and my players, but also for the footballing community of Japan as a whole.
"I think objectively we can say with certainty that the overall level of the Japan players now is much higher than in the past.
"As for how far we can go, we keep challenging and keep going forward - that's all."
Paraguay are in the Round of 16 for the fourth time in their history, having achieved the feat in 1986, 1998 and 2002.
Midfielder Cristian Riveros is excited by the prospect of reaching the quarters, but hopes to go even further in the tournament.
"I guess we're cool, we're at ease. The important thing is to get to the quarters," he said.
"But we must go bit by bit and if we want to go any further we have to go through the various steps.
"If we make it we will certainly go down in Paraguay's history, so we would like to take another step forward, and then another after that."