At Vitrua tennis touches on above... Nike have basically all their eggs in two baskets, Federer and Nadal. Everyone else is an afterthought and borderline irrelevant in terms of global marketing efforts.
They don't take punts of players by paying them millions of dollars - they offer them deals in the 5 or low 6 figures and sometimes with added performance incentives. When someone gets offered a deal by another brand - as Isner, Monfils and Del Potro have shown - Nike seems to not give a hoot basically. There's always another bus like them coming along. They're waiting for the next Federer or Nadal.