With the advent of 30- to 40-m class ground-based telescopes in the mid-2020s, direct imaging of exoplanets is bound to take a new major leap. Among the approved projects, the Mid-infrared Extremely Large Telescope (ELT) Imager and Spectrograph (METIS) instrument for the ELT holds a prominent spot; by observing in the mid-infrared regime, it will be perfectly suited to study a variety of exoplanets and protoplanetary disks around nearby stars. Equipped with two of the most advanced coronagraphs, the vortex coronagraph and the apodizing phase plate, METIS will provide high-contrast imaging (HCI) in L-, M- and N-bands, and a combination of high-resolution spectroscopy and HCI in L- and M-bands. We present the expected HCI performance of the METIS instrument, considering realistic adaptive optics residuals, and investigate the effect of the main instrumental errors. The most important sources of degradation are identified and realistic sensitivity limits in terms of planet/star contrast are derived.