Japan were focusing firmly on a possible place in the World Cup quarter-finals after holding off 14-man Samoa 28-22 in Toulouse on Thursday.
Samoa were thinking of the 2027 World Cup.
The Brave Blossoms, classified as a second tier rugby team, need to beat Argentina, who are tier one, to advance to the last eight for a second straight World Cup.
"Today was a Test match we had to win to earn the right to play for a place in the quarter-finals," said Japan coach Jamie Joseph.
"For a tier two team you reach the quarter-finals you have to beat a tier one team, but you also have to bowl over two motivated tier two teams and that's what we did tonight."
Both Argentina and Japan have already lost to Pool D's other tier-one team England, who are sure of advancing after Thursday’s result.
"It is knockout footy for us, we have got to try beat a tough Argentina team next week," said Japan full-back Lomano Lemeki.
"We put a lot of emphasis on this game but it is just a first step. We need to take it on to Argentina next week."
Even though they are eliminated, coach Seilala Mapusua said Samoa want to beat England in their last match to finish third and avoid having to qualify for the 2027 World Cup in Australia.
"We'll take the next few days to recover well, we have a few injuries. Then we will worry about England," he said.
When Japan captain Kazuki Himeno scored his team's third try after 49 minutes just after Samoa's Ben Lam was sent off, the Brave Blossoms seemed in control. But two late tries brought Samoa within a score before Japan held on.
"It was a really tough game and we had to work bloody hard to get the win. Hats off to the Samoan boys – they really took it to us," said Joseph.
"They are big fellas aren't they? They got on top of us at times and we had to really keep at it for the whole game. The commitment was certainly there but there were one or two things to work on for next week."
Himeno said he was proud.
"I am really proud of the boys. The effort and hard work of everyone is what won just the game," said Himeno.
Samoa edged Japan 24-22 in Sapporo in July. Both coaches felt the difference was the way the red cards were dealt.
"It was an arm wrestle," said Joseph. "The game was very even. In Sapporo we had the red card. They had the red card today."
Mapusua agreed, saying: "It's tough playing a game against 15 men and to be down a man, with the red card. Unfortunate for Ben, it was just one of those contacts in the game.
"Japan exploited the advantage and they got the win."
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