Magnetic vortices and antivortices are usually observed coexisting in cross-tie walls within ferromagnetic thin films. An antivortex can be isolated by utilizing the shape anisotropy. The isolation offers the possibility to investigate its fundamental dynamics as a two-dimensional oscillator in a confining potential. Hitherto, the gyration mode of vortices and its excitation has been studied intensely. Here, a detailed investigation of the coupling of an antivortex to in-plane rotating magnetic fields is presented. The resonant response is imaged by time-resolved scanning transmission x-ray microscopy to determine the amplitude as well as the phase of the gyration. The experimental results are compared with the analytical model of a two-dimensional harmonic oscillator derived from the Thiele equation. As a cause for deviations from the model, a buckled energy landscape due to local defects is discussed.