Defocusing particle tracking velocimetry (DPTV) is applied to sub-millimetre rotor-stator gap flows on a test rig of an open wet clutch model. A recently proposed in-situ calibration approach (Fuchs et al., 2016) is modified to account for both the rotating no-slip condition and the grooves of the clutch lamella. Fluorescent particles are used to suppress the reflection issues at the walls and the groove patterns. The results demonstrate that DPTV is capable to derive sufficiently accurate velocity information in the smooth Couette-like gap region. Moreover, the experiments uncover formerly unknown flow properties inside the lamella grooves. The results provide evidence that the application of DPTV is a promising means to achieve a deeper insight into cause-effect relations between the flow fields inside an open wet clutch and the resulting adverse drag-torque characteristics.