knife-edge

noun2

1. •
   A small piece of metal, usually steel, sharpened to an acute edge or angle, and resting on a smooth surface, serving as the fulcrum of a pendulum, scalebeam, or other piece required to oscillate with minimal friction.
2. •
   (figurative) A precarious balance that could be upset by a very small force in either direction.

   • The game was on a knife-edge.