Former Japan captain and head coach Hirao – nicknamed "Mr Rugby" in his homeland – died three years ago this Sunday aged 53 after a battle with cancer.
Full-back Ryohei Yamanaka played under Hirao, who represented the Brave Blossoms at the 1987, 1991 and 1995 World Cup, at Kobe Steelers.
"[The quarter-final] is the date he passed away, so there'll be a game on an important day for me as well," he said.
Japan's scrum coach Shin Hasegawa was handed his international breakthough by Hirao during his playing days.
"I'm a bit emotional talking about Hirao," he added. "He was the one who picked me for the national team, he was the one who played me. We have a game on a special day. I hope we can pay him back.
"The best memory is receiving a letter in my room a day before our opening match in the 2003 World Cup. It wasn't that long but had things that encouraged me and made me feel, 'I need to fight for this man'.
"I remember heading into the game with a good motivation. I asked him one time why I was chosen and he said, 'For the scrum, of course', so I really focused on it. He really kept his eyes on me and was a great coach."
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Japan stunned the Springboks with a 34-32 victory at the 2015 World Cup and hooker Atsushi Sakate explained they are leaving no stone unturned in pursuit of a repeat – combining brain and brawn.
"Our psychologist, Dave [David Galbraith] makes quizzes and writes them on the whiteboard," he explained.
"It is part of the training focusing on how to use your brain under pressure and in tough situations.
"It was started at our training camp in Abashiri. You have to make decisions during the toughest time during the match. You use your brain.
"That is why it was put up when we had tough weightlifting training."
Photo Getty Images Caption: "Mr Rugby" Seiji Hirao