We introduce a new method to apply controlled forces on single molecules. The motion of a micron-sized bead attached to a solid surface through a single molecular contact is tracked by evanescent wave microscopy as a force is exerted through a flow. We report measurements of the streptavidin-biotin bond rupture force obtained with this technique. We also obtain detailed measurements of the balance of forces involved in detaching an adhering bead with a flow. A small lateral force translates into a much bigger normal force on the attachment point. This effect is relevant for the interpretation of common cell adhesion assays.