Saracens v Toulon (Sunday 3pm)
It could come down to marksmanship, but with Owen Farrell and Jonny Wilkinson perfectly capable of cancelling each other out from the kicking tee, it is more likely that the most talked-about Heineken Cup semi-final in years will be decided in the more abstract areas of the game: desire, resilience, collective identity, that kind of thing.
Saracens were once considered the most ambitious headhunters in European rugby – they hired Michael Lynagh, Philippe Sella and Francois Pienaar at the start of professional era; they mined the rich seam of talent in South Africa in laying the foundations for their current side – but Toulon surpassed them three years ago and are out-recruiting them still, by a distance. One of the visitors' English captures, the open-side flanker Steffon Armitage, disagrees violently with the view that he and his colleagues are merely "a bunch of rock-star mercenaries", but until they land a major title, the description will stick to them like glue.
Many thought Armitage would be one of the key figures at Twickenham tomorrow, but Bernard Laporte, the former France national coach who runs the show on the Cote d'Azur, has relegated him to the bench. The Springbok giant Danie Rossouw and the Argentine maestro Juan Fernandez Lobbe will start on the flanks, with the All Black forward Chris Masoe between them. It is not the worst, as back-row trios go. Equally intriguing is Laporte's choice in the engine room, where Bakkies Botha, pretty much as scary as ever, will be joined by the former London Irish lock Nick Kennedy. This selection has patently been driven by Steve Borthwick's presence in the Saracens pack. Borthwick remains the most effective line-out plotter around, so Kennedy, no slouch himself, is there purely to hold things together in this crucial department.
For all their lavishly financed raids on foreign rugby strongholds – only four of their starting line-up are French – there is no guarantee that Toulon will match Saracens' unity when the tough questions are asked. Heaven knows, they have the personnel to win the game and advance to next month's final in Dublin. What nobody knows is if they have the attitude.
C Auvergne v Munster (Saturday 5pm)
Talking of attitude, Munster have it in spades when it comes to this competition. Their quarter-final victory at Harlequins, driven primarily by the 2009 Lions captain Paul O'Connell and his allies in the back five of the pack, was a timely reminder of the Irish province's depth of feeling when it comes to the Heineken Cup.
They will need that emotional charge and more to prevail in southern France today against everyone's tip for the title, but one look in the eyes of O'Connell and his principal henchmen, most notably the ferociously competitive blind-side flanker Peter O'Mahony, will at least tell the favourites that they are in a game. Munster have made one change following their grand victory in London, the Lions back Keith Earls returning to the right wing.
Leinster v Biarritz (Saturday 2.30pm)
Right from the start of the season, Leinster were confident of playing knock-out rugby this weekend. Unfortunately for the reigning European champions, it is not in the competition that matters most. Even so, they are taking this second-tier semi in Dublin with all due seriousness: their main contenders for Lions selection – Rob Kearney, Brian O'Driscoll, Jonathan Sexton and the rest – are present and correct.
Can Biarritz conceivably win? It is a tall order. There again, the marvellous scrum-half Dimitri Yachvili won this title for them last year, against a Toulon side no one expected them to beat. And Yachvili plays.