Integrated writing is increasingly used in language assessment programmes. As a hybrid task, it requires students to coordinate different language skills, i.e. listening, reading and writing, to retrieve information from multiple sources, and compose an essay for a specific purpose. Tapping into the varied skills that contribute to successful integrated writing is especially beneficial for low proficiency students. However, the mechanisms underlying the impact of these skills on integrated writing performance have yet to be thoroughly studied. This study sampled 103 first-year undergraduate students in Hong Kong who showed relatively low proficiency in Chinese language. They completed three independent tasks measuring their listening, reading, and writing skills; an integrated listening-reading-writing task; and an integrated writing strategy use questionnaire. The results indicated that together, the independent skills accounted for 29.5% of the variance in integrated writing performance, suggesting that integrated writing is a skill that goes far beyond the simple combination of listening, reading, and writing. Independent writing showed the strongest correlation with integrated writing, while both independent listening and independent writing exerted direct and indirect effects on integrated writing performance. However, the effect of reading on integrated writing performance was insignificant, even though the two were significantly correlated. These results offer insights into the complex relationships between the skills. The findings enrich our understanding of the construct of integrated writing, as well as suggest strategies for teaching less proficient learners.