The Kenyans vs. the Ethiopians.
Like many distance events at the Rio Games, and Olympics past, it often boils down to a race featuring the indefatigable runners from these two African neighbors.
And that's a likely scenario in the women's marathon this Sunday, and the men's next Sunday, the final day of the Summer Olympics.
In every men's and women's marathon since 1996, at least one Kenyan or Ethiopian has made it to the medal stand, with one exception. And that exception proves the rule.