Based on the concept of "learner agency", this single case study examines the self-instructed language learning process of a language learner with ChatGPT since May 2024. To serve the purpose of the study, qualitative data were collected through a semi-structured interview, the participant's chat logs with the bot, and his learning diary. The analysis of collected data revealed (1) factors that led to the learner's informed decision to incorporate ChatGPT as a primary learning tool into his self-practice for an international standardized English test, (2) how the tool was utilized to serve different learning purposes, and (3) the main challenges the learner encountered during that process. The findings provide implications for language learning approaches with transformative artificial intelligence (AI) technologies while emphasizing the importance of open discourse on AI's integration into education and of training on self-study skills for learners.