Wafer-scale single-crystalline graphene monolayers are highly sought after as an ideal platform for electronic and other applications(1-3). At present, state-of-the-art growth methods based on chemical vapour deposition allow the synthesis of one-centimetre-sized single-crystalline graphene domains in similar to 12 h, by suppressing nucleation events on the growth substrate(4). Here we demonstrate an efficient strategy for achieving large-area single-crystalline graphene by letting a single nucleus evolve into a monolayer at a fast rate. By locally feeding carbon precursors to a desired position of a substrate composed of an optimized Cu-Ni alloy, we synthesized an similar to 1.5-inch-large graphene monolayer in 2.5 h. Localized feeding induces the formation of a single nucleus on the entire substrate, and the optimized alloy activates an isothermal segregation mechanism that greatly expedites the growth rate(5,6). This approach may also prove effective for the synthesis of wafer-scale single-crystalline monolayers of other two-dimensional materials.