Ireland's opening win over Scotland and New Zealand's victory over South Africa had many already penciling in a Ireland-South Africa quarter-final.
But that script was torn to shreds on Saturday after Japan ran out shock 19-12 winners over Joe Schmidt's side in Shizuoka to take control of Pool A.
Ireland have a quick turnaround before they travel to Kobe to take on Russia, knowing they could still face the Springboks in the last eight, but results need to go their way.
How can Ireland top Pool A?
The first priority of course is getting two bonus point wins over Russia and Samoa in the remaining fixtures. This will give Ireland another 10 points for a final tally of 16 points.
Two victories for Japan over Samoa and Scotland without any bonus points would give them 17 points to top the pool.
But Ireland will be hoping Scotland can do them a favour and beat Japan in the final round of fixtures. A Scottish win, in these circumstances, would see Ireland top the pool.
And Japan would be knocked out of the World Cup on the head-to-head result with the Scots - providing our Celtic neighbours get a bonus point win over Russia in their final Pool game.
Two bonus point wins for Ireland though would mean that Scotland can't knock them out of the World Cup as Gregor Townsend's side can only get to 15 points.
But 10 more points for Ireland and Japan beating Samoa and Scotland, will see the Brave Blossoms top the pool and consign Ireland to a quarter-final date with New Zealand.
The worst case scenario is that Ireland don't bag bonus point wins over Russia and Samoa, leaving them open to being knocked out by Scotland and Japan.
But, in that instance, a Japan victory over Scotland would be the favoured result for Ireland, leaving us to face the Kiwis.
Then again, predicting results in this group has already been shown to be very risky business and Russia and Samoa might fancy causing a few upsets of their own.
A lot of ifs and buts...
Results and Fixtures
Fri 20 Sep Japan 30-10 Russia (Tokyo)
Sun 22 Sep Ireland 27-3 Scotland (Yokohama)
Tue 24 Sep Russia 9-34 Samoa (Kumagaya)
Sat 28 Sep Japan 19-12 Ireland (Shizuoka)
Mon 30 Sep Scotland 34-0 Samoa (Kobe)
Thu 3 Oct Ireland v Russia (Kobe), 11.15am
Sat 5 Oct Japan v Samoa (Toyota), 11.30am
Wed 9 Oct Scotland v Russia (Shizuoka), 8.15am
Sat 12 Oct Ireland v Samoa (Fukuoka), 11.45am
Sun 13 Oct Japan v Scotland (Yokohama), 11.45am
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