  Measurements are made of differential cross-sections of highly boosted
pair-produced top quarks as a function of top-quark and $t\bar{t}$ system
kinematic observables using proton--proton collisions at a center-of-mass
energy of $\sqrt{s} = 13$ TeV. The data set corresponds to an integrated
luminosity of $36.1$ fb$^{-1}$, recorded in 2015 and 2016 with the ATLAS
detector at the CERN Large Hadron Collider. Events with two large-radius jets
in the final state, one with transverse momentum $p_{\rm T} > 500$ GeV and a
second with $p_{\rm T}>350$ GeV, are used for the measurement. The top-quark
candidates are separated from the multijet background using jet substructure
information and association with a $b$-tagged jet. The measured spectra are
corrected for detector effects to a particle-level fiducial phase space and a
parton-level limited phase space, and are compared to several Monte Carlo
simulations by means of calculated $\chi^2$ values. The cross-section for
$t\bar{t}$ production in the fiducial phase-space region is $292 \pm 7 \
\rm{(stat)} \pm 76 \rm{(syst)}$ fb, to be compared to the theoretical
prediction of $384 \pm 36$ fb.
