one horseman; single rider
Refers to a single mounted warrior or rider, often in historical or military contexts. Commonly appears in the phrase 一騎打ち (single combat).
They decide the outcome in single combat.
He rides alone into the enemy camp.
一気 (いっき) means 'in one go/without stopping', while 一騎 (いっき) is unrelated and means 'one mounted warrior/horseman'.
Compound of 一 (one) and 騎 (horseman, rider). The reading いっき is a regular sound change from いち + き.