Surgery may involve precise instrument tip positioning in a minimally invasive way. During these operations, the instrument is inserted in the body through an orifice. The movements of the instrument are constrained by interaction forces arising at the orifice level. The physical constraints may drastically vary depending on the patient's anatomy. This introduces uncertainties that challenge the positioning task for a robot. Indeed, it raises an antagonism: On one side, the required precision appeals for a rigid behavior. On the other side, forces applied at the entry point should be limited, which requires softness. In this paper we choose to minimize forces at the orifice by using a passive ball joint wrist to manipulate the instrument. From a control perspective, this leads to consider the task as a 3 DOF wrist center positioning problem, whose softness can be achieved through conventional low impedance control. However, positioning the wrist center, even with a high static precision, does not allow to achieve a high precision of the instrument tip positioning when the orifice behavior is not known. To cope with this problem, we implement a controller that servos the tip position by commanding the wrist position. In order to deal with uncertainties, we exploit an adaptive control scheme that identifies in real-time the unknown mapping between the wrist velocity and the tip velocity. Both simulations and in vitro experimental results show the efficiency of the control law.