Adidas has effected an impressive turnaround of late, making it once again a legitimate rival to Nike. But it still lags Nike in key areas — most notably basketball, where Under Armour's Curry line has given the smaller company considerable market share. A combined Adidas-Under Armour would give NKE a legitimate competitor for the first time in decades. At the least, it would all but ensure what essentially would be a worldwide duopoly in athletic apparel and footwear.