Supervision lies at the heart of research-based doctoral education. Existing scholarship has recognized the role of supervision in students' academic socialization and identity construction but presented little empirical evidence based on prolonged observations of actual supervisory interactions. Addressing this gap, the present study adopted a digital micro-ethnographic approach to examine how a supervisor mediated a doctoral student's academic identity construction in their supervisory interactions. Multiple-sourced data were collected over a two-year span, including the focal participants' interactions through instant messaging, interviews, and artefacts. Informed by a sociocultural view of identity work in social interaction, data were subjected to both thematic analysis and micro-level discourse analysis. The analyses revealed that the supervisor drew upon "transformative criticism," "strategic recognition," and "his own identity" to mediate the doctoral student's academic identity construction in an iterative process that often involved rounds of negotiations between different positionings. Situating the supervisor's discursive practice at the macro-sociological level of doctoral education, this study contributes new insights into doctoral supervision as a hybridized and contingent practice that integrates the professional and the personal as well as the hierarchal and the collegial. The study offers important implications for doctoral supervision.