The formation of the most massive O-type stars is poorly understood. We present a case study of a young massive clump from the ATLASGAL survey, G328.2551-0.5321. It exhibits a bolometric luminosity of 1.3$times$10$^4$ L$_{odot}$ corresponding to a current protostellar mass of $sim$11 and 16 M$_{odot}$. We analyze high angular-resolution observations with ALMA at $sim$0.17 corresponding a physical scale of $sim$400 au in dust continuum and molecular lines. The dust continuum emission reveals a single high-mass protostellar envelope and shows evidence for a marginally resolved continuum source. We detect a rotational line of CH$_3$OH within its $v_{rm t}$=1 torsionally excited state revealing two bright peaks of emission spatially offset from the dust continuum peak, and exhibiting a distinct velocity component $pm$4.5 km s$^{-1}$ offset compared to the source $v_{rm lsr}$. Local thermodynamic equilibrium analysis suggests N(CH$_3$OH)=1.2$-$2$times$10$^{19}$ cm$^{-2}$, and kinetic temperatures of 160$-$200 K at the position of these peaks. Their velocity shifts correspond well to the expected Keplerian velocity around a central object with 15M$_{odot}$ consistent with the mass estimate based on the sources bolometric luminosity. We propose a picture where the CH$_3$OH emission peaks trace the accretion shocks around the centrifugal barrier, pinpointing the interaction region between the collapsing envelope and an accretion disk. Because the HC$_3$N $v_{rm 7}$=1e ($J$=38-37) line shows compact emission, and a velocity pattern consistent with models of Keplerian rotation, we suggest that this could be a new tracer for compact accretion disks around high-mass protostars. The estimated physical properties of the accretion disk suggest a specific angular momentum several times larger than typically observed towards low-mass protostars.
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