Higher education programs are often characterized by lecture-style knowledge transfer and criticized for not preparing learners for authentic professional situations. Recently, the use of Approximations of Practice to simulate real-world professional demands has been discussed as promising in two ways. First, simulations can be used to assess competence in a situated way close to performance yet in a controlled setting. Second, simulations allow to practice applying acquired knowledge in authentic, yet not overwhelming settings, thus supporting competence development. This contribution presents the design of a video simulation of diagnostic one-on-one interviews to assess and foster pre-service mathematics teachers’ diagnostic competence in the area of decimal fractions. The design of the video simulation followed three criteria: perception of authenticity, presence, and low extraneous cognitive load. Descriptive results show that participants’ perceptions of the video simulation differ only weakly from those of a parallel role-play simulation. An exemplary implementation of the video simulation in a regular university course is presented.