upper hand

noun

A dominating position, as in a conflict: advantage, better¹, bulge, draw, drop, edge, superiority, vantage. Informal inside track, jump. See over/under.

Also, whip hand. A dominating or controlling position, as in Once you let Jeff get the upper hand there'll be no stopping him, or When it comes to checkers, my son-in-law generally has the whip hand. The first term alludes to an ancient game in which each player in turn grasps a stick with one hand, beginning from the bottom, and the last who can put his hand at the top wins. Its figurative use dates from the late 1400s. The variant alludes to the driver who holds the whip in a horse-drawn vehicle; it was being used figuratively by the late 1600s.

The noun has one meaning:

Meaning #1: position of advantage and control
  Synonym: whip hand