The paper discusses the effect of a full-height opening in the in-plane performance, under lateral loading, of a masonry infill wall partitioned with horizontal sliding joints. The innovative infill construction technique, which was already presented in previous works, is here tested implementing the details required by the presence of an opening. The specimen was designed with a vertical post located at the opening side, which is meant to protect the opening fixtures (window or door) from the infill sub-panels' relative sliding. The results showed the efficiency of the post in governing the sliding mechanism between the infill sub-portions and the role of the post's stiffness in modifying the in-plane response of the infill. Thanks to the post's deformability, the overall infill-frame interaction was mitigated, as testified by the reduction of the specimen lateral strength with respect to previously tested similar infills without openings. The role of such deformability was explored by modifying the post's stiffness with additional steel profiles coupled to the initial wooden post configuration, in different test phases. The direct measuring of the shear action at the post ends and of its deflection provided useful information for the design of the post and for the future calibration of numerical models.