Direct liquid-cooled end-pumped thin-disk lasers, which offer high output potential for beam quality, were analyzed focusing on addressing thermally induced wavefront distortion issues. A hydrodynamic laminar flow model was utilized for this purpose. Comparing the temperature distributions and wavefront distortions under two thermal loading conditions ideal full aperture pumping and less-than-ideal nonfull aperture pumping-revealed that heat accumulation within the coolant along the flow direction causes wavefront distortion along the same direction. In additional, a sharp decrease in temperature at the pumping edge position along the transverse direction of the thin-disk leads to considerable deterioration in the laser wavefront. Experimental results corroborate these findings. Furthermore, our thermal safety assessment highlights that the maximum thermal load of a single thin-disk is constrained by thin-disk stress, which becomes the bottleneck before the temperature rise at the solid-liquid interface. The analysis of thermally induced wavefront distortion revealed that the thermal deformation of the Nd∶YLF thin-disk has a minimal effect on wavefront distortion and the primary source of wavefront distortion is the fluid thermo-optic effect.