Single point diamond turning (SPDT) of large functional surfaces on silicon remains a challenge owing to severe diamond tool wear. Recently, tremendous efforts have been made in understanding the machining mechanics, especially wear mechanism of diamond tools in SPDT of silicon. However, the localised transition of machining mode from ductile to brittle as a result of progressive tool wear has not been well understood yet. In this paper both experimental and numerical simulation studies of SPDT were performed in an effort to reveal the underlying phenomenon of ductile to brittle transition (DBT) as a consequence of diamond tool wear. Series of facing and plunging cuts were performed and the profile of machined surface was evaluated together with the progression of tool wear. The transition stages from ductile to brittle were identified by analysing the surface profiles of plunging cuts using a scanning electron microscope (SEM) and a 2D contact profilometer and a white light interferometer. The progressive degradation of the cutting edge of diamond tool and its wear mechanism was determined using Least Square (LS) arc analysis and SEM. The study reveals that at initial tool wear stage, the ductile to brittle transition initiates with the formation of lateral cracks which are transformed into brittle pitting damage with further tool edge degradation. Numerical simulation investigation using smoothed particle hydrodynamics (SPH) was also conducted in this paper in order to gain further insight of variation of stress on the cutting edge due to tool wear and its influence on brittle to ductile transition. A significant variation in frictional resistance to shear deformation as well as position shift of the maximum stress values was observed for the worn tools. The magnitude and distribution of hydrostatic stress were also found to change significantly along the cutting edge of new and worn diamond tools.