The emergence of ChatGPT, an OpenAI chatbot, has revolutionised our education system to a new phase. It is an innovative approach to future learning, including in the English as a Second Language (ESL) classroom. Since its inception, ChatGPT has been remarkably used by most educators and learners. Nevertheless, its application in the flipped classroom, especially in enhancing ESL learners' speaking skills, is notably an innovative and novel phenomenon. Besides, educators' challenges in giving instant feedback to learners and creating an engaging learning environment in the flipped classroom emerged as a crucial consideration. Therefore, this study aims to investigate the effectiveness of ChatGPT as a partnering tool in developing ESL learners' speaking skills in the flipped classroom setting in a Malaysian public university. This study also scrutinises ESL learners' challenges in using ChatGPT to develop speaking skills. The researchers employ a mixed-method design incorporating a five-point Likert scale questionnaire and focus group interviews. The quantitative data was analysed using descriptive statistics, while the qualitative data was analysed through thematic analysis. The findings of this study offer significant implications for various stakeholders in the educational industry. Policymakers, course designers, educators, learners, and even the Ministry of Education will obtain valuable insights, especially in decision-making, innovative teaching approaches, and learners' speaking abilities in the ESL flipped classroom.