Japan coach Shogo Mukai blamed his team's tough World Cup schedule following their 41-13 loss to Fiji.
Thursday's defeat in Townsville came just four days after a 51-29 loss to France.
Japan now have just three days to prepare for their final Pool B match against the United States in Gosford.
Mukai said: "The players could hardly move in the two days after the
French match, so we had only a couple of days to prepare for this game.
"It's hard for players to prepare mentally or physically for a match like today.
"Now we have only three days before the match with the USA so it could be very difficult again."
Japan gave a good account of themselves in the first half but the Fijians took control after the interval.
"The first half of the game was as we planned and we played well," said Mukai.
"In the second half, especially, Fiji used long kicking and our forwards had to run backwards. That made all the players' stamina run out.
We were exhausted.
"When we tried to go into Fiji's territory, we made lots of mistakes and gave penalties away and those kind of things contributed to the big score."
Japan's New Zealand-born fly-half Andrew Miller added: "We failed to match (Fiji's) intensity.
"Communication wasn't there right from the kick off, when we had a mix up. It got worse as the game went on."
Having six players in his squad who worked for Japanese clubs had helped Fiji coach Mac McCallion determine the strengths and weaknesses of his rivals.
But he said: "I'm not here to judge Japan, but I think they have played very well here and have improved dramatically in the last 12 months."
Fiji's five-try haul will give them confidence ahead of the 1 November match against Scotland that will determine quarter-final berths from Pool B.