This paper contributes to research on teaching excellence by extending the current body of literature pertaining to mentoring pedagogies in undergraduate research settings across diverse social, institutional and disciplinary contexts. Our data comes from in-depth interviews with 32 international faculty who have received excellence awards for undergraduate research mentoring. The data reveal a freedom -- control dialectic, illuminating the ways in which expert mentors negotiate the desire to create opportunities for students to experience freedom and creativity in research, yet maintain control over the topic, quality and outcomes. The research findings reveal a defining characteristic of award-winning mentors as an ability to establish and sustain a sense of challenge, while maintaining meaningful engagement and a sense of achievement amongst students. The findings show the importance of tailoring practice to the needs of particular student groups, and there are implications for institutional resourcing, as well as mentor training and development.