• Japan have won their past four World Cup games • We have to think we are starting at zero, says captain Leitch

There has been a routine question asked of the leading nations this week: how do you rate Japan’s prospects after the victory over Ireland? It showed how one result can radically transform opinion and expectation, and the New Zealand flanker Sam Cane carefully fielded it by saying: “It will be exciting to see how they go against Scotland.”

Japan have to overcome Samoa first in Toyota on Saturday and a danger after their thrilling if, to the outside rugby world at least, unexpected victory over Ireland is that attention not openly leap-frogs to the game against Scotland at Yokohama a week on Sunday, which closes the group stage, but regards the quarter-finals as beneath the summit for the home side.

On Monday a local television news channel was casting for overseas journalists to answer a number of questions, one of which was whether Japan can make the final. The victory over Ireland lit up the tournament, igniting imagination and idealism and sending temperatures higher than they are in the various locations where the tournament is being played.

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Japan are on a World Cup roll, winning their last four matches in the tournament, and five out of six, having succeeded once in their first 24 attempts, against Zimbabwe in 1991. Their run started against Samoa in Milton Keynes almost four years to the day, but just three of the side that started that day will take the field this time.

They include the flanker Michael Leitch, the team captain who will not be in charge this time as Pieter Labuschagne, who was given the role against Ireland, remains in the job. It was the introduction of Leitch on the half-hour that helped turn a match Ireland had been in control of, his clear-headedness and direct running helping soothe excitability into composure.

Japan’s stark improvement in the past two World Cups has been a direct result of their being awarded this one. It gave them access to the championship countries in both hemispheres: the Brave Blossoms played all 10 between 2016 and last month, unprecedented for an emerging nation, and for the past four years had a presence in Super Rugby through the Sunwolves. A Japan team that in past tournaments conceded 145 points to New Zealand, 91 to Australia and 72 to Wales is now equipped to cope.

Leitch, the captain in 2015, is the face of the team in advertisements and posters here. Before the tournament, he was asked about Japan’s goal and his answer was revealing. “I think you need to be very brave in what you say,” he said. “We want to make the quarter-finals, but ideally we want to win the World Cup. That is an outrageous statement but it changes your whole behaviour. We are going to have to try to win it and see how far we get.”

Japan had prepared for Ireland for more than a year, targeting the fixture when many thought Scotland – and finishing second in the group – would be their priority. “It is all about being mentally prepared,” he added this week. “We have to think that we are starting at zero after last week and just think about Samoa. They lost their last game and will be playing with pride. We need to steel ourselves and have an even better mentality.”

In one sense, it will be a more significant match than Ireland. It will show how Japan are able to cope with the hype that has blown up around them and their opponents, who lost 34-0 to Scotland on Monday, know one more defeat will send them home. “Japan are in our way, just another team we have to beat to get to where we want to be,” said their Bristol back-rower Chris Vui. “We are hugely motivated.”