PurposePower is one of the single most critical concepts for understanding industrial relations management. Yet, despite having been subject to much scholarly attention, existing research often implicitly assumes an organizational level of behavior or examines influence tactics within the confines of an individual-level dyad. This has led to a limited understanding of influence exercises involving a third person. Motivated to advance the understanding of this phenomenon, this study aims to explore how boundary spanners from a buying organization influence supplier representatives by involving a third person.Design/methodology/approachThis study uses a longitudinal single case study design grounded in social control theory. Data consist of interviews, observations and documents collected over a period of 27 months in a global sourcing context.FindingsThe findings demonstrate how the influence agent from the buying organization can invoke a third person’s mediated and nonmediated social power base through either direct or indirect social control mechanisms. With these findings, this paper makes a novel theoretical contribution by developing a deeper understanding of underexposed social influence tactics unfolding in individual-level triads.Practical implicationsThis study offers boundary spanning managers with practical insights into subtle and indirect forms of social influence with which they can influence external exchange partners.Originality/valueTo the best of the authors’ knowledge, this study is the first to draw on social control theory to examine interpersonal influence tactics in buyer-supplier relationships. By integrating this theoretical perspective with extant research on social power, this study emphasizes the importance of considering how the influence agent mobilizes a third person’s social influence base.