When optimally tuned, a tuned mass damper (TMD) can be very effective for reducing the response amplitude of a structure or mechanical system when it is excited near resonance. If the stiffness of the TMD were to develop nonlinear properties, either due to operation outside of its linear range or component degradation, its ability to function can be significantly compromised. Previous work has demonstrated that semi-active tuned mass dampers (STMDs) are able to provide better vibration attenuation than passive devices with significantly lower power requirements than active devices. In this paper, the authors numerically explore the use of an STMD to address the undesirable conditions resulting from a TMD developing nonlinear (cubic) hardening properties. It is found that by adding the STMD in series to the structure/nonlinear TMD (NTMD) system, the response of the NTMD can be reduced, limiting its motion to the linear range where it is able to better attenuate the response of the primary structure.