Research on impact of teacher beliefs on their practices has been recognized as one of the important aspects in the discipline of mathematics education. This study reports the results of a case study that gives an insight about the influence of professed beliefs of pre-service secondary mathematics teachers on their instructional practices in the Sri Lankan context. The pre-service teachers' professed beliefs were examined by using a questionnaire of six-point Likert scale items. Data on instructional practices were collected through classroom teaching observations and follow-up post-lesson interviews. Qualitative analysis of the audio-taped classroom teaching observation transcripts was performed, using a list of sensitizing concepts that reflected flexible and rigid beliefs aspects. The results reveal that professed beliefs encouraged them to adopt flexible practices, but to differing extents due to the influence of social expectations and contextual demands embedded within this educational context.