The future of tidewater glaciers in response to climate warming is one of the largest sources of uncertainty in the contribution of the Greenland Ice sheet to global sea level rise. In this study, we investigate the ability of an ice sheet model to reproduce the past evolution of the velocity and surface elevation of a tidewater glacier, Upernavik Isstrøm, by prescribing front positions. To achieve this, we run two ensembles of simulations with a Weertman and a regularised-Coulomb friction law. We show that the ice flow model has to include a reduction in friction in the first 15 km upstream of the ice front in fast-flowing regions to capture the trends observed during the 1985-2019 period. Without this process, the ensemble model overestimates the ice flow before the retreat of the front in 2005 and does not fully reproduce its acceleration during the retreat. This results in an overestimation of the total mass loss between 1985 and 2019 of 50 % (300 Gt vs. 200 Gt). Using a variance-based sensitivity analysis, we show that uncertainties in the friction law and the ice flow law have a greater impact on the model results than surface mass balance and initial surface elevation.