Hifumi Abe overcame world champion Joshiro Maruyama in a one-off bout to secure the final place in Japan's judo team for their home Tokyo 2020 Olympic Games.
The duo had been expected to go head-to-head at the All-Japan Weight Class Championships, which would have determined the Japanese men's under-66 kilogram representative.
The coronavirus pandemic led to the event being postponed, however.
A one-off contest between the pair was arranged at the famed Kodokan venue, with coverage streamed online to an audience of around 400,000.
Abe emerged as the winner in the golden score period, following a 20-minute bout.
He twice required treatment during the clash, after sustaining a cut to his left finger and suffering a nosebleed.
The two judokas both received two shido penalties, leaving them a further sanction away from losing the contest.
Abe, a two-time world champion, clinched victory with an inner leg sweep to achieve the golden score.
He will now join his sister Uta in the Japanese team at the postponed Olympic Games.
"The match was really long but I remember every scene," Abe said, according to the Olympic Channel.
"A lot of emotions flowed through me over the last several days but today, I just had to go out there and do it.
"It was do or die.
"I had the will and never doubted myself for a moment throughout the match.
"Now I can officially say I'm aiming for a gold medal with my sister at the Olympics.
"As her bigger brother, I can't afford to lose."
Abe joins Naohisa Takato, Shohei Ono, Takanori Nagase, Shoichiro Mukai, Aaron Wolf and Hisayoshi Harasawa as Japan's judo representatives in the men's weight divisions.
Funa Tonaki, Uta Abe, Tsukasa Yoshida, Miku Tashiro, Chizuru Arai, Shori Hamada and Akira Sone have secured nominations for the women's events.
Japan has won more Olympic gold medals in judo than any other sport, securing 39 to date.
The country earned three Olympic titles at Rio 2016 in the sport, with 12 of their 14-member squad leaving the Games with a medal.