Japan beat South Africa 34-32 with a try in extra time to claim a victory which has been described as the biggest upset in World Cup history.
Former Highlanders coach Greg Cooper, who coached in Japan for more than five years, was in Tokyo over the weekend and said Japan had worked extremely hard over the past four years to get where it is now.
''I think it is a surprise that they beat the Springboks but not a surprise with the quality of the performance,'' Cooper said.
''They have improved a lot. They've got a good scrum, a good maul ... the standard of rugby now in the top league in Japan is very, very high.
''These teams have high-quality coaches; guys who have coached at Super rugby, international coaches who have helped improve the game over there.''
He said the top company teams in Japan would be the equivalent standard of ITM Cup sides with high-quality foreign players mixing with Japanese players.
Japan coach Eddie Jones had worked very hard over the past four years to get a team which could achieve at the World Cup.
''Eddie had to compete at the set piece. Worked hard on the scrum. He got a scrum coach over from France [Marc del Maso] who the Highlanders used.
''Then he got Steve Borthwick from England to work on the lineout. There has always been good strength and conditioning coaches up in Japan.
''They had to be strong in the contact area. The Japanese are not big people but he set out to get the most well-conditioned rugby team he possibly could. He knew he was going to get heart from them.''
Rugby was still a niche sport in Japan. Baseball and sumo wrestling were still very popular.
The game was popular at universities and Cooper said crowds of up to 50,000 watched those clashes. Away from university, the other top sides were company sides.
Former All Black Paul Miller played in Japan for more than five years and said the influx of foreign players and coaches had really helped the national team succeed.
''It was surprising but in some ways it was not. Just the foreign influences that have made a difference. They are creating and instilling habits into Japanese players,'' Miller said.
''They used to see it as a job where it was very much regimented. Used to see playing and training as a task but now it is more about enjoyment and having fun.''
He said the Japanese had been set in their ways but this had changed with the influx of foreign players and coaches.
