Implausible as it may seem, Roger Federer and Rafael Nadal share precisely the same height and weight: 1.8m, and a rigorously controlled 85kg.
It is some freak of genetic divergence that two such antithetical specimens could ever be sculpted from the same raw material and yet their rivalry, which reaches its 33rd and perhaps final defining instalment in Melbourne tonight, has always been derived less from their common ground than their exotic differences. Where Federer is more comfortable with Anna Wintour or Credit Suisse suits, Nadal would far rather kick back with his closest amigos over the PlayStation in Manacor.
The parallels run no deeper than the Nike ticks on their respective bandanas. The polarities, however, help to define their endlessly engrossing duels: innate gift versus unflinching resolve, Swiss understatement versus Latin flamboyance, groomed metrosexual versus piratical brute. As Jon Wertheim observed in Strokes of Genius, his account of their 2008 Wimbledon final, this was perhaps the ultimate juxtaposition of artist and conquistador. Or perhaps Nadal was just playfully accentuating the contrast in that sleeveless top.
As sport's most exquisite treat, the Federer-Nadal dynamic is a delicacy we could not have dared dream we would still be relishing. Having reached its finest vintage five years ago, its lustre faded just slightly last season as the Spaniard administered fearful thrashings in Indian Wells, Rome and London, heightening the impression that his nemesis was drifting - with supreme elegance, naturally - into the night. One lone three-set tussle in Cincinnati was not enough to dispel suspicions that the balance of power had swung too far for this to be considered the greatest contest.
Federer's restoration in Australia, under the gaze of fellow understated aesthete Stefan Edberg, has shown such conclusions to be premature.
The only pity, as a result of his fall to No6 in the rankings, is that he is not seeded to meet in Nadal in the final.
Instead, we must accept that the final - whether an all-Swiss collision of Federer and Stanislas Wawrinka, or a rerun of Nadal's 2010 Wimbledon triumph at the expense of Tomas Berdych - will have a distinct after-the-Lord-Mayor's-Show flavour next to today's banquet.
The on-court/off-court dichotomy in their relationship is another compelling draw. Their murderous confrontations in Grand Slams soften into mutual respect the second they pack their racquets away.
Privately, though, their battles have moved them to tears. Nadal was a broken man after his five-set Wimbledon defeat by Federer in 2007, while after the 2009 Australian Open the Swiss player mumbled through heaving sobs: "Oh, man, it's killing me."
This time, Nadal seeks to reaffirm his supremacy as world No1 with a third slam title in four attempts, while Federer looks to sweep away a few autumn leaves aged 32 with an 18th major, his first since 2012. So if there is scope for a diversion tonight, take a moment to savour this reunion of an odd couple. The privilege is all ours.