The Rugby World Cup exploded into life on Saturday as Japan produced one of the biggest upsets in the tournament’s history on home turf, stunning the highly fancied Ireland 19-12.

The result throws Pool A wide open, as Japan took a huge stride towards their first-ever quarter-final from a group that also includes Scotland.

In the earlier World Cup game, Argentina got their campaign back on track after an agonising opening loss against France, beating a spirited Tongan side 28-12 with the help of a hat-trick from hooker Julian Montoya.

And South Africa routed Namibia as expected, also bouncing back after their loss to defending champions New Zealand in their tournament opener to record a 57-3 victory.

Kenki Fukuoka struck the killer blow for hosts Japan, ensuring they take a giant step towards a first rugby World Cup quarter-final spot.

The replacement winger wriggled over on the hour mark to score the winning try of a frenetic Pool A clash as the Brave Blossoms added the “Shizuoka Shock” to the “Brighton Miracle” after toppling South Africa at the 2015 tournament.

Japan coach Jamie Joseph insisted the result had not come as a surprise.

“You don’t want to come across too arrogant and cocky,” said the ex-All Black.

“We’d been preparing for this game for a hell of a lot longer than they had,” added Joseph.

“We were preparing for three years and Ireland probably since Monday, so we felt like had an advantage. The boys will have a couple of beers tonight.”

Ireland coach Joe Schmidt, meanwhile, denied that his side had underestimated Japan.

“It’s a tough defeat for us to take,” he admitted. “But I’d like to congratulate Japan for the energy, intensity and skill they brought to the game.

“Japan didn’t exceed my expectations -- I expected them to be as good as they were. They play a quality game and they’re very difficult to contain.”

“Anyone that is utterly shocked hasn’t seen how good they are,” said Ireland captain Rory Best.

“We were on the wrong side of the penalty count, but Japan posed a lot of questions to us and unfortunately we couldn’t come up with the right answers.”

Ireland, who thrashed Scotland 27-3 in their opening game, had looked in control after first-half tries from Garry Ringrose and Rob Kearney.

But they went off the boil as Japan grew into the match, roared on by a partisan crowd of 47,000. A late inclusion on the bench, Fukuoka made the difference, darting over on the overlap to complete a stunning team try and put the home side in front.

Incredibly, they stayed there, leaving Ireland’s players slumped on the turf in disbelief.

Kotaro Matsushima, hat-trick hero of Japan’s 30-10 win over Russia in last week’s tournament curtain raiser, posed an early threat on as Japan edged a cagey opening. The Irish had come into the tournament ranked the best side in the world.

The Springboks meanwhile ran in nine tries against their fellow Africans, a mis-match that became even more pronounced when the Namibians went down to 14 men towards the end of the game, the second yellow card of the match.

The Brave Blossoms will now target the Scotland match, the last game of the pool phase, to shore up their route through to a first-ever quarter-final, with their next game against Samoa, on a short turnaround, on Oct.5.

Scotland were poor by their own admission against the Irish and coach Gregor Townsend changed his entire backrow for their now critical clash against the Samoans on Monday.

On their showings so far, Japan will fancy their changes against Townsend’s men, who made a litany of errors in their drubbing by Ireland at a rainy Yokohama Stadium.

Elsewhere, Argentina’s bonus-point win could prove crucial in the World Cup’s “group of death” that also includes England and France.

As coach Mario Ledesma rang the changes after they went out to a 28-0 lead, Argentina lost some of their momentum and the spirited Pacific islanders fought their way back into the game, scoring two tries and finishing the game the stronger.

Agence France-Presse