  Measuring the neutrino mixing parameter \ensuremath{\sin^2\theta_{13}} to the
sub-percent precision level could be necessary in the next ten years for the
precision unitary test of the PMNS matrix. In this work, we discuss the
possibility of such a measurement with reactor antineutrinos. We find that a
single liquid scintillator detector on a reasonable scale could achieve the
goal. We propose to install a detector of $\sim10$\% energy resolution at about
2.0~km from the reactors with a JUNO-like overburden. The integrated luminosity
requirement is about 150~${\rm kton}\cdot {\rm GW}\cdot {\rm year}$,
corresponding to 4 years' operation of a 4~kton detector near a reactor complex
of 9.2 GW thermal power like Taishan reactor. Unlike the previous $\theta_{13}$
experiments with identical near and far detectors, which can suppress the
systematics especially the rate uncertainty by the near-far relative
measurement and the optimal baseline is at the first oscillation maximum of
about 1.8~km, a single-detector measurement prefers to offset the baseline from
the oscillation maximum. At low statistics $\lesssim 10$~${\rm kton}\cdot {\rm
GW}\cdot {\rm year}$, the rate uncertainty dominates the systematics, and the
optimal baseline is about 1.3~km. At higher statistics, the spectral shape
uncertainty becomes dominant, and the optimal baseline shifts to about 2.0~km.
The optimal baseline keeps being $\sim 2.0$~km for an integrated luminosity up
to $10^6$~${\rm kton}\cdot {\rm GW}\cdot {\rm year}$. We have assumed that the
TAO experiment will improve our understanding of the spectral shape
uncertainty, which gives the highest precision measurement of reactor
antineutrino spectrum for neutrino energy in the range of 3--6~MeV. We find
that the optimal baseline is $\sim 2.9$~km with a flat input spectral shape
uncertainty provided by the future summation or conversion methods' prediction.
