A standard procedure in processing vertical seismic profile (VSP) data is the separation of up- and downgoing wavefields. We show that these wavefields in boreholes can be retrieved using only single-component data, given that a full set of surface reflection data is also available. No medium parameters are required. The method is an application of the Marchenko method and uses a focusing wavefield. It is a wavefield that satisfies certain focusing conditions in a reference medium. We show that the method is applicable to boreholes with any orientation, and no receiver array is required. By this work, we present two contributions. One is that we investigate the effect of using only the traveltime from borehole data to form the focusing wavefield. The second is that we validates standard separation methods (PZ summation and f-k filtering) by retrieving the one-way wavefields from a completely different approach. We use the numerically modelled data from a realistic field velocity model in the North Sea. Three borehole geometries (horizontal, deviated and vertical) are tested. We discuss the practical aspects for field application in the end.