The 802.11ac amendment of the IEEE 802.11 standard, known as a very high-throughput wireless local area network (WLAN), has attracted extensive attention for achieving a maximum transmission rate up to 7 Gb/s. This is mainly possible by adopting the DownLink MultiUser Multiple-Input-Multiple-Output (DL MU-MIMO) technology, which allows multiple frames to be simultaneously transmitted from the access point (AP) to different receivers via multiple spatial streams. To support the DL MU-MIMO technology at medium access control level, a technique called transmission opportunity (TXOP) sharing has been proposed to extend the mandatory TXOP of the WLAN standard. In this letter, we propose an analytical model based on Markov chains to evaluate the performance of an IEEE 802.11ac AP enabling the TXOP sharing mechanism. Based on this model, we are able to derive the achievable throughput of a given access category (AC), namely, voice (VO), video (VI), best effort (BE), and background (BK). The ultimate goal of this letter is to analyze how the TXOP sharing mechanism could improve the utilization of the scarce wireless bandwidth while achieving channel access fairness among the different ACs. Note that the obtained theoretical results are also validated by computer simulation