This study investigates how graduate teaching assistants (GTAs) differ from regular graduates in terms of input characteristics (i.e., who they are), process characteristics (i.e., how they experience the PhD trajectory), and the self-estimated likelihood of successfully completing the PhD. Additionally, it assesses to what extent and how the input and process characteristics explain the self-estimated success rate between the two groups. The data come from four waves of the PhD Survey (2018, 2019, 2020 and 2021; N = 1,766) conducted at a large university in Brussels (Belgium). Results show that GTAs estimated their likelihood of successful completion of their PhD lower compared to regular graduates. This difference is mediated by a lower satisfaction with the supervisor support and a higher amount of time pressure among GTAs. Additionally, GTAs' surplus of time spent on teaching duties and the lack of a research plan was negatively related to the self-estimated likelihood of successful completion to a greater extent than regular graduates.