Saturday 23 September is a date that has been writ large in the diaries of South Africa and Ireland rugby fans for a very long time.

Having been drawn in the same pool as each other for the first time in Rugby World Cup history, the match at Stade de France was always going to be a big occasion in its own right.

However, with the teams currently in first and second place in the World Rugby Men’s Rankings powered by Capgemini ahead of kick-off, it has an added edge.

South Africa will end Ireland’s 14-month reign at the top if they win with Andy Farrell’s men dropping to third below France if Les Bleus do what everyone expects them to do and beat Namibia.

However, Ireland will hold on to the number one ranking if they draw with South Africa – albeit only by 0.09 rating points.

A win would see Ireland’s cushion over South Africa grow to as much as 6.05 rating points if the margin is by more than 15 points.

If beaten, South Africa will also slip below a victorious France side into third.

Defeat for France will see them drop below New Zealand, who do not play this weekend, into fourth place. However, Les Bleus can fall no further than that.

Shock defeats will be costly for England and Scotland

Like France, England and Scotland are unable to improve their ratings in victory over Chile and Tonga respectively. England’s rating is 22.75 points better than Chile’s, while Scotland’s rating is 13.14 points higher than Tonga’s.

Scotland and England, therefore, will remain fifth and sixth with victories. However, England could move above Scotland if they win and Scotland lose, the latter potentially falling by as many as five places to 10th depending on the margin and other results.

A defeat could see England drop to a new low of ninth, depending on other results.

Accurately reflecting the rankings, Pool C is the most open as things stand with Wales, Fiji and Australia all vying for a top-two finish that guarantees a place in the quarter-finals.

Those three teams occupy seventh to ninth in the rankings and are covered by just 1.66 points, meaning the result of the Wales v Australia match in Lyon could have major repercussions.

A draw between Wales and Australia will see Fiji climb one place to equal their best position of seventh at the expense of the Welsh.

If that happens in tandem with defeats for both England and Scotland, Fiji could jump as high as fifth.

Australia will fall to a new low of 10th if they lose their Pool C meeting with Wales and Argentina beat Samoa in Saint-Étienne.

However, victory for the Wallabies could see them jump as high as fifth in the event that England and Scotland both suffer shock defeats, as well as keeping their hopes of maintaining their ever-present quarter-final record alive.

Eight-year high within Samoa's grasp

Meanwhile, defeat for Samoa will see them drop two places to 13th if Italy and Georgia beat the lower-ranked Uruguay and Portugal respectively.

A win for Samoa will lift them to ninth, climbing above both Argentina and the loser of the Wales-Australia match. Manu Samoa have not been ranked as high as ninth since August 2015.

Portugal will become the higher ranked of the two nations for the first time since March 2006 if they beat Georgia in Toulouse on Saturday.

Os Lobos have not beaten their rivals since February 2005 but a new high awaits them if they can record their first-ever Rugby World Cup win.

Portugal could climb as high as 13th depending on the margin of victory and the results of other matches.

Georgia will fall one place, below Japan, if held to a second draw in as many years by Portugal – two if Tonga shock Scotland.

The Lelos can only improve on 13th place in victory if Italy fail to beat Uruguay or Samoa are beaten by Argentina.

If both lose then Georgia will climb to 11th, the record-equalling position they were in when the tournament kicked off.

Historic win would lift Los Teros

Uruguay must beat Italy for the first time in five attempts, and by more than 15 points, to become the higher ranked of the two nations, a scenario that could see them equal their highest position of 14th.

Defeat for Italy will see them fall between two and four places, depending on the margin and results of teams around them.

A win for Tonga against Scotland will see them climb above at least Japan, with the Brave Blossoms not playing this weekend. Any further gains will depend on margins of victory and other results.

Namibia will return to the top 20 for the first time since June 2017 if they can end their Rugby World Cup duck with victory over France.

Chile, meanwhile, will enter the top 20 for the first time if they can upset England and claim their first Rugby World Cup win and Namibia fall to Les Bleus.