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Nanoscale sliding friction involving a polystyrene melt near its glass transition temperature Tg (373 K) exhibited dissipation phenomena that provide insight into the underlying molecular relaxation processes. A dissipative length scale that shows significant parallelism with the size of cooperatively rearranging regions (CRRs) could be experimentally deduced from friction-velocity isotherms, combined with dielectric loss analysis. Upon cooling to ∼10 K above Tg, the dissipation length Xd grew from a segmental scale of ∼3 Å to 2.1 nm, following a power-law relationship with the reduced temperature Xd∼T R-[lowercase_phi_synonym]. The resulting [lowercase_phi_synonym]=1.89±0.08 is consistent with growth predictions for the length scale of CRRs in the heterogeneous regime of fragile glass formers. Deviations from the power-law behavior closer to Tg suggest that long-range processes, e.g., the normal mode or ultraslow Fischer modes, may couple with the α relaxation, leading to energy dissipation in domains of tens of nanometers. [ABSTRACT FROM AUTHOR]