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.
The Springboks 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. But it was the Japan-Ireland contest that everyone was waiting for although even the most partisan home fan could not have predicted that the hosts would turn over their illustrious Six Nations opponents.
Ireland had polished their credentials as one of the tournament favourites by beating Scotland 27-3 in their opener but the defeat to Japan leaves their qualification for the knock-out stages uncertain. 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 October 5.
Meanwhile, 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.
Saturday's final match saw the Springboks take on their African rivals Namibia at the City of Toyota Stadium. South Africa were hoping to bounce back after their opening day loss to treble-chasing New Zealand and racked up a sizeable score against Namibia, the lowest-ranked team in the tournament.