Wales is in second place in Group B with three points, and their fate depends on their result against Russia and England against Slovakia
The thousands of Welsh supporters who have brought their nation's famed singing, not to mention plenty of plastic dragons, to a major tournament after a wait of more that half-a-century are steeling themselves to hit the heights -- or rock bottom -- on Monday.
Currently in second place in Euro 2016 group B with three points, Wales could finish either top or bottom of their section, depending on their own result against Russia here and England's against Slovakia.
If they beat the Russians, who have only one point, and neighbours England fail to win, Chris Coleman's side would be proud group winners, qualifying for a round of 16 match in the Parc des Princes next Saturday against a third-placed team.
At the other extreme, defeat, and at least a point for Slovakia, would leave them fourth.
A modicum of pride would undoubtedly remain, but such an outcome would be a real anti-climax after the excitement of beating the Slovaks 2-1 in the opening game and then holding England until the final minute in a 2-1 defeat.
Russia's 2-1 defeat by Slovakia undid all the good work of their resilience in snatching a late equaliser against England and leaves them vulnerable to an early return home and charges of under-achievement.
In historic and geo-political terms there should be no contest between a nation of 144 million people and one with a population of three million -- larger than only Albania, Northern Ireland and Iceland of the 24 countries here.
Russia have never lost to Wales in their four meetings, but the margins have been tight, like so many matches at this low-scoring tournament.
Coach Leonid Slutski knows his team must avoid conceding free kicks within shooting range for Gareth Bale, a mistake that both Slovakia and England were punished for making.
The Real Madrid player scored with direct long range efforts, bringing back memories of Michel Platini's ability with a dead ball when France last hosted the European Championship in 1984.
Coleman knows he can count on his one superstar. He will be more concerned about a defence that conceded only four goals in the qualifying campaign but has been beaten almost as many times in two games here.
Changing from his system of three at the back is a possibility, but perhaps too radical a departure at this late and crucial stage.