That includes the USA, twice victorious against Japan at World Cups but now several paces behind in international rugby.
A third of the country tuned in for the Brave Blossoms' game against Samoa -- the fixture that followed the 19-12 victory over Ireland -- and with the knockout stages looming, rugby fever continues to sweep through Japan.
"I think that's what all of us want," United States captain Blaine Scully tells CNN Sport of Japan's win against Ireland.
"It's good for the game, it's good for the rugby world, the rugby landscape ... they're serving as an incredible example of the growth of a rugby country both on the field and through the experience they've been able to provide with the Rugby World Cup."
For a team that was being thrashed 145-17 just over two decades ago, Japan's transformation from plucky underdog to potential quarterfinalist is certainly heartening, and the USA, which is thought to have ambitions of hosting the Rugby World Cup in 2027, can look to the Japanese for encouragement.
"I think (the US hosting the tournament) would be a dream come true for a lot of us. You think what it would be like running out of the tunnel in an American city representing the United States at the Rugby World Cup -- it would be a really special experience.
"For us it's making sure that we focus on the nuts and bolts of growing American rugby to really get the maximum impact that hosting the Rugby World Cup could have as a catalyst and a catapult to the future."
The Eagles lost their first three games against England, France, and Argentina, but now has its best chance of victory yet against Tonga. The chances of a quarterfinal berth gone, the team's last pool stage game is about confidence and pride.
But there have been promising signs this World Cup. At one point in the second-half, the US trailed France by just three points; it was three late tries that produced the lopsided 33-9 scoreline.
Scully stresses that he and his teammates can "compete with anybody" on their day, and the France game proved just that.
"We want to win and we want the result," says the wing, who is a veteran of three World Cups. "But it's fundamentally dictated by the quality of the process and your performance.
"We're hugely focused on what we can control and our application to what we do day-in, day-out and what we take out of each training session."
Now 31 and with the 2023 World Cup already scheduled to take place in France, it's unlikely Scully will get the chance to play in a World Cup on home soil.
But the introduction of Major League Rugby in 2017 -- the USA's latest effort to establish a professional competition -- at least affords the chance to play in the US.
Scully says he is "definitely" interested in playing club rugby in MLR having announced he would be leaving Cardiff Blues at the end of last season, although he qualifies that it's "probably a conversation with my wife in the future and whatever she wants me to do."
The focus now, though, is on the game against Tonga and trying to end the World Cup on a high note. Deciding what happens beyond that will have to wait 80 minutes.