The best clubs in Europe like it when the last matchday of a Champions League group stage feels almost like an exhibition game. Plan your campaign right and after five, even four, fixtures qualification for the knockouts should be sealed, and you can ease up in December.
Barcelona and Juventus, like Bayern Munich, Liverpool, Manchester City and Chelsea are already through, and the only remaining argument for Barca and Juve, first and second in Group G, is who finishes top.
But doing the arguing are the two most celebrated footballers of the century. No exhibition has a better pair of names than Lionel Messi and Cristiano Ronaldo.
It has been two and half years since their last head-to-head, a 2-2 draw when Messi was guiding Barcelona to a Liga title and Ronaldo was focused on the last of his Champions League successes with Real Madrid, ahead of joining Juventus.
Thirty-six Messi v Ronaldo duels earlier, they faced off for the first time, in the first leg of the 2008 Champions League semi-final, Barcelona against Manchester United, with whom Ronaldo went on to lift his first European Cup.
Camp Nou will be crowd-free, because of public health considerations, for bout 37 in their epic rivalry. It is a rivalry that has shared out 11 of the last 12 Ballons d'Or – Messi is one ahead – and nine of the last 15 European Cups – Ronaldo 5, Messi 4.
And yet you can't help but view with a hint of nostalgia. Tuesday night may well be 33-year-old Messi’s last duel with Ronaldo, 35, in which the younger man is wearing a Barcelona jersey. His contract expires in June and Barca, ninth in La Liga, are not making a strong case to persuade him their standards still match his ambitions.
Messi won his last Champions League in 2015. Juve bought Ronaldo because their last one dates back to 1996. The Portuguese has not yet delivered Juve that sought-after prize, but can be in no doubt his former clubs still miss him. Next week’s draw for the last-16 stage could yet have no place at all for Madrid and United.
Madrid have a do-or-die assignment in topsy-turvy Group B on Wednesday. United find themselves wedged in a three-way neck-and-neck tussle for the two top spots in Group H, up against clubs who, in Ronaldo's time at United, would scarcely have been considered their equals in Europe. But Paris Saint-Germain and RB Leipzig have risen fast. They were contesting a Champions League semi-final less than four months ago.
United, on nine points, are at Leipzig on Tuesday needing to avoid defeat to finish in one of the top two places. Second-placed PSG – nine points – need one more to guarantee progress and are at home to Istanbul Basaksehir, whose only interest is preserving their pride.
The ominous news for United is that Leipzig, who will definitely go through to the last 16 with a victory, have goals in them: the 3-3 draw with Bayern at the weekend means they have scored nine times in their last three fixtures.
The good news for Ole Gunnar Solskjaer, the United manager, is that he is in charge of a squad entitled to believe they travel as confidently as anybody.
United won their ninth Premier League away game in succession at West Ham, the 3-1 victory one of five in which they have come from behind. This United, capable of brittleness and brilliance within the blink of an eye, also beat PSG at the Parc des Princes in October, and followed up by putting five unanswered goals past Leipizg at Old Trafford.
The trouble for Solskjaer is that his United have another, very distinct persona: their defeat in Istanbul, and PSG's victory over 10-man United, after Fred's dismissal in Manchester has left the group perilously poised. "We never make it easy for ourselves," Solskjaer said. "But these are the games we look forward to."
Edinson Cavani and Athony Martial, suffering “minor niggles”, have not travelled but their manager insists that, even without two senior strikers, the intention in Germany is to target all three points. “You can’t sit back and hope for a 0-0 draw,” said Solskjaer. “It’s not in our genes.”