Japan's first international foe may have been Canada, and its most regular opponent South Korea, but it is Tonga with whom it has the closest rugby relationship, a rapport that would never have existed without one man's love of the abacus.
Five players, either born in the Pacific island or of Tongan descent, have been included in Japan's 23-man squad for Saturday's game against the 'Ikale Tahi, and this week the team received a lecture from captain Michael Leitch about the bond between the two countries.
The relationship was forged in 1976 when the then monarch of Tonga, King Taufa'ahau Tupou IV, was introduced to the "soroban," or Japanese abacus, by Toshio Nakano, a former manager of Daito Bunka University's rugby team.
The king's fascination with the counting machine allowed long-standing links to be established between his nation and the university, creating a pathway into Japan for Tongan rugby players.
Sinali Latu, who represented Tonga in two test matches in 1984, was among the first to study there.
In 1987, two years after arriving in Japan, Latu debuted for his adopted country and went on to win 32 caps before retiring and becoming a Japanese citizen.
(Tonga captain Siale Piutau)
"Latu competed in three World Cups, and had a huge impact on Japanese rugby," Leitch said of the 53-year-old, whose son Kruger currently plays for Panasonic Wild Knights.
Leitch highlighted one game in particular, the 1990 clash between Japan and Tonga in Tokyo, as an example of the impact the former No. 8 had.
"The head coach at the time said in an interview that he was worried whether Latu could compete against his home country with his normal fighting spirit. But when he asked Latu, he was told 'it's no problem at all' and that he shouldn't be worried because 'I have loyalty toward the Japanese national team.' Latu played in the game, and was outstanding in a 28-16 win."
Leitch -- who is half Fijian and half New Zealander and who went to high school in Sapporo before attending Tokai University -- said the legacy left by Latu did not just have an impact on Tongans coming to Japan, but on all foreign-born players who ended up studying and playing rugby in Japan.
"If he didn't come, I don't think a player like me would be around. If he didn't succeed, we wouldn't see all these foreign students coming over. The legacy he left behind is huge and wonderful."
The friendly relationship between the two countries was also evident this week off the field with a couple from Kochi Prefecture, where Tonga will be based before the Rugby World Cup, donating kit to the Pacific islanders after theirs went missing en route from Fiji.
"We had some issues with luggage when we got here so they flew us up some T-shirts and training shirts," said Tonga coach Toutai Kefu, whose side includes Asahi University graduate Sione Vailanu, currently playing in England for European champions Saracens.
'Ikale Tahi captain Siale Piutau, who played for Yamaha Jubilo from 2012 to 2016, explained the unique relationship from a Tongan point of view.
"The similarities in terms of two cultures is the big one, in terms of respect and honor that the Japanese have for one another. We all enjoy coming here for those reasons as it feels like we are back home in Tonga."