Wood-fibre conformation affects paper properties in various paper-product categories, such as packaging, printing, and absorbents. Qualitative investigations suggest that capillary forces play a crucial role in determining the fibre conformation upon drying. To quantify this process, we theoretically and experimentally investigate deformation of a circular tube under capillary pressure. Fibre pit holes, which impose a significant capillary pressure while drying, are modelled as circular holes in a tube. The calculations are undertaken by coupling the analytical solution for buckling a circular tube to numerical solutions of the Young-Laplace equation. This elasto-capillary model elucidates the influences of wetting angle, tube-wall flexibility, and hole size on the tube deformation. In experiments, flexible silicon-rubber tubes with a hole in the wall are filled with liquid, and the internal pressure is measured while withdrawing this fluid, thus mimicking evaporation during the drying of wood fibres. The results prove that capillarity can collapse the fibre lumen, either partially or completely, depending principally on the fibre-wall flexibility.