This afternoon's tie between Biarritz and Bath pitches the most ambitious of French clubs against the least consistent of English ones - teams who know so little about each other they might almost be described as perfect strangers. Tomorrow's thunderous all-Ireland affair between Leinster and Munster is quite the opposite. Their match is a back-garden scrap between next-door neighbours.
The French champions, among the favourites for the title from the outset, make no bones of the fact that they consider the West Countrymen to be one of life's mysteries. Who can blame them? Brian Ashton feels the same way, and he is the Bath coach. Yet if Ashton cannot explain the peaks and troughs of form that still beset his charges, apart from to suggest that his efforts to open his players' minds to new thinking necessarily involves a "suck it and see" element, he relishes this unexpected opportunity for European advancement precisely because of the unfamiliarity factor.
"From my perspective," he said yesterday, "this situation suits me down to the ground. It's the kind of challenge I most enjoy, plotting to play against a team who don't quite know what to expect from us. If we make mistakes in this game, Biarritz have the players to punish us. But this is the time for mental courage. I'd be worried if any team under my direction went into a match reluctant to play some rugby, and I'll be bitterly disappointed if that happens in a game of this magnitude on one of the best stages imaginable. As I keep saying, if you can't get excited about an occasion like this, why bother?"
Ashton is most worried about the potency of the Basques' kicking game - a game they play with extreme prejudice through the likes of Dimitri Yachvili, Julien Peyrelongue, Damien Traille, Philippe Bidabé and Nicolas Brusque. "All French teams have this in their armoury nowadays," he said. "We beat Bourgoin twice in the pool stage, but still lost out by 30 metres every time there was a kicking duel. We'll have to be extra-careful this time."
Over in the Irish capital, the Munster hordes descending on Lansdowne Road for their team's sixth semi-final in seven years are wondering whether Ronan O'Gara can put boot to ball with similarly destructive effect. This is a classic confrontation between the piano players and the piano shifters, with Munster cast in the latter role. If O'Gara kicks poorly, Brian O'Driscoll and his colleagues in a wonderfully accomplished Leinster back division will cause a dozen different kinds of mayhem. If O'Gara has one of his on-days and presents his roughhouse forwards with the field positions they relish, not even Michael Cheika, the highly regarded Leinster coach, will have an answer.
Lansdowne Road is not Thomond Park - for which O'Driscoll and company are profoundly grateful, for the Munster pack is close to unstoppable when driven along by a Limerick crowd that continues to set new standards of rabid zealotry.
Leinster will feel far more secure in tomorrow's surroundings. Munster know how to play them, and the Dubliners know they know. But by beating Toulouse in the way they did in the quarter-final at the start of the month, the great underachievers of Irish rugby proved, to themselves as much as anyone, that they are capable of staring down opponents of the highest class on an occasion of the utmost significance.
Neither tie is easy to pick. Biarritz are probably the most complete of the four contenders, but they tend to play so within themselves that any flash of self-expression seems an aberration. They have the all-round strength to cope with Bath, just as Munster have the muscle to deal with Leinster, but this tournament deals in the currency of surprise. Only a soothsayer or a fool would lay down a penny of his money at the betting shop.