  The sensing capability of a single optically bright electronic spin in
diamond can be enhanced by making use of proximal dark nuclei as ancillary
spins. Such systems, so far only realized in bulk diamond, provide orders of
magnitude higher sensitivity and spectral resolution in the case of magnetic
sensing, as well as improved readout fidelity and state storage time in quantum
information schemes. In nanodiamonds, which offer additional opportunities as
mobile nanoscale sensors, electronic-nuclear spin complexes have remained
inaccessible. We demonstrate coherent control of a 13C nuclear spin located
4{\AA} from a nitrogen-vacancy center in a nanodiamond and show quantum-state
transfer between the two components of this hybrid spin system. We extract a
nuclear-spin free precession time of T2* = 26 us, which exceeds the bare
electron free precession time in nanodiamond by two orders of magnitude.
