Japan went into half time leading the Rio Olympic gold medalists.
Fiji started well and withing less than a minute, Jiuta Wainiqolo ran straight through two Japanese defenders to score.
Iosefo Masi scored the team’s second try.
However, just before halftime Japan managed to score a team try through Lote Tuqiri as the hosts led 14-12 at the break.
Another Fijian, Kameli Soejima got Japan further in the lead with an unconverted try just after halftime as they led 19-12.
Fiji tried to put a few phases together but successive mistakes from Sireli Maqala and Aminiasi Tuimaba didn’t help their cause but their defensive was sound which resulted in a try to Waisea Nacuqu.
Nacuqu later ran in another try for a 24-19 lead with less than two minutes to play.
Replacement player Asaeli Tuivuaka was sent to the bin for what the referee deemed as a deliberate knock on.
The defending champions went on to win their first match against Japan 24-19.
Fiji’s second match will be against Canada at 8pm (local time) tonight.
The final group game will be held tomorrow at 12;30pm against Great Britain.