Japan's incredible upset over Ireland, in a repeat of their Brighton miracle at the last Rugby World Cup, has thrown the All Blacks' quarter-final opponent wide open.

Technically the All Blacks are yet to qualify for the quarter-finals but having repelled the Springboks and with Canada, Namibia and Italy in the coming weeks the greatest upset in World Cup history would be required to prevent Steve Hansen's men topping their pool.

Who the All Blacks will face in the first knockout round, though, promises to be a fascinating subplot.

As it stands it could be any one of Japan, Samoa, Ireland or Scotland.

Ireland's 27-3 opening victory over Scotland seemingly set Joe Schmidt's side up to top Pool A and avoid the All Blacks in a quarter-final clash but the dominant nature of that result now seems a distant memory.

This 19-12 defeat relegates Ireland to second place in their pool – three points behind Japan. Joey Carbery kicking the ball out to end the match appeared a defeatist attitude but the bonus point Ireland secured by finishing within seven points of Japan could yet prove crucial to hopes of progression.

Ireland have Samoa and Russia to play and will hope to secure bonus points from those matches but their fate, and that of who the All Blacks will face in their quarter-final, won't be decided until Scotland meets Japan in the final pool match on October 13 in Yokohama.

This poses potential challenges for the All Blacks as they may now have less time than previously expected to analyse their quarter-final opponent.

Should Japan defeat Scotland, the All Blacks may meet Ireland in a heavyweight quarterfinal.

Equally, though, Samoa and Scotland could beat Japan and Ireland may top the pool.

With any number of possibilities in play suspense will build until kickoff in that final pool match.

Japan were incredibly nervous and mistake-riddled in their win against Russia to open their home tournament but after Jamie Joseph and Tony Brown masterminded this victory over Ireland they should now harness immense confidence for the remainder of their pool matches.

"We've been focusing on today for the last year at least, probably three years subconsciously, and Ireland have been thinking about it since Monday," Joseph told media post match.

"All teams go to a pattern under pressure. When we got ahead, Ireland tried to take us on physically. We were prepared for that. The players showed real courage. It's those moments in big games of rugby that win the game. I'm proud of our boys."

Sitting top of their pool with nine points from two games, Japan are on the verge of qualifying for the knockout rounds for the first time in history, a dynamic that is sure to grip the host nation.

The emotion at the final whistle from the Japanese players was replicated in the stands and at fan zones around the country.

The width and pace Japan employed troubled Ireland throughout second half in particular. In the first half alone Japan threw 119 passes to Ireland's 63. Japan then contested every breakdown and defended with supreme line-speed.

Once in the lead, they held their nerve to deliver another World Cup shock that doubles down on their win over the Springboks in Brighton four years ago and proves, on their day, they can foot it with rugby's long-established elite.

This result served a reminder than no one can be taken for granted and emphasised why this is the most open World Cup we have witnessed.

Chasing the game is not one of Ireland's strengths. Already without influential injured playmaker Johnny Sexton, who watched on from the stands, as injuries decimated Ireland's backline late in the second spell they were made to look ponderous.

Schmidt's ashen-face expression post match said everything about a performance that must rank among the worst of his era. The shock result now leaves Ireland in a scrap to make it out of their pool, and potentially on course for a quarter-final showdown with the All Blacks.

World Cups by nature are unpredictable and it's worth remembering France made the final in 2011 despite losing to Tonga and the All Blacks in pool play.

Such history, though, will be of little consolation to Schmidt.