When taking a sample of liquid contained in a flexible closed container, a problem to be resolved relates to piercing the wall of the container with a non-cutting piece, whilst obtaining a precise and clean opening for accessing the liquid and preventing any unintended flow or spill of liquid. This problem is resolved by a flexible bag for liquid, particularly a urine or blood bag containing a device for taking a sample of liquid from said flexible bag. The sampling device has a circular opening, arranged so that this opening is accessible from a first side of the device. The device also has a support structure on its other side, said support structure with at least one support element, whose function is to distribute forces, and which is positioned at a distance from the plane of said opening.
