Catalyzes the reduction of the keto moiety of phthiodiolone dimycocerosates (DIM B) and glycosylated phenolphthiodiolone dimycocerosates to form the intermediate compounds phthiotriol and glycosylated phenolphthiotriol dimycocerosates during phthiocerol dimycocerosates (DIM A) and glycosylated phenolphthiocerol dimycocerosates (PGL) biosynthesis. Belongs to the mer family. Phthiodiolone/phenolphthiodiolone dimycocerosates ketoreductase subfamily. According to PubMed:15995190, this enzyme is not functional in M.kansasii, due to mutations outside this fragment, possibly in the ribosome-binding site or promoter/regulatory regions.