The purpose of this study is to examine which, and how, reflective practices as instructional strategies affect students' self-directed learning (SDL) competencies in second language university classes. To answer the research questions, an explanatory mixed-methods approach was used to extend quantitative research through qualitative research. Based on the 28-item SDL Competency Scale, a survey was conducted of students enrolled in one of two different classes titled "Advanced Listening & Speaking" and "Public Speaking" in Japan. A total of 25 students participated in the survey. In addition, 11 students were interviewed for in-depth student perceptions of reflective practices, and a thematic analysis was used to analyse the interview data. The survey results indicated that reflective practices were significant predictors of SDL competencies. Among the three reflective practices of collaboration, self-reflection, and peer feedback, collaboration was the most significant predictor of SDL competencies. Based on the findings from the interviews, we also developed a thematic map which presented the specific relationships between reflective practices and SDL competencies. The results of this study provide significant implications regarding effective reflective practices to help students become self-directed learners in second language university classes.