Considering flow as a non-ergodic process (i.e. non-homogeneous across indi- viduals and non-stationary over time) that happens at the within-individual level, in this research we work with Bakker's model that propose flow as made up by three components: intrinsic motivation, enjoyment, and absorption. Taking into account that flow theory can be considered as an intrinsic motivation theory, and the recent proposals about the need to distinguish between pre-conditions of flow and the flow experience itself, we look at interest as a moderator between the challenge/skills balance and the experience of flow, rather than a component of the flow experience. A total of 3640 recordings were collected from a sample of 58 workers using an experience sampling method (several registers a day, during 21 working days). The data was analyzed using regression techniques in each participant (i.e. at within-individual level). Our work tries to respond to the following two research questions: Will interest play a moderating role in the relationship between challenge/skills balance and flow? Will a non-linear model (cusp catastrophe model) better explain the relationship among challenge/skills balance, interest, and flow? The results suggest that our hypotheses were correct: including interest as moderator better explains the relationship between challenge/skills balance and flow in comparison to a model without moderation (R 2 values change from 0.33 to 0.50). Additionally, carrying out the analysis following non-linear techniques explained more variance as well (R 2 = 0.67), and this increment was significant. These results support the idea that interest should be considered as a key precondition for the appearance of flow, and this relationship is non- linear. We could say that these findings are exemplary in the field and brings up questions for their application in further research.