IEEE 802.11 distributed coordination function supports two access mechanisms - the basic access and the four-way access, both of which are vulnerable to the hidden and the exposed node problem (collectively called the concealed node problem). Though most of the works in literature suggest to overlook this problem due to the associated overhead to solve them, this paper shows that the problem becomes severe for high speed wireless mesh networks. An opportunistic four-way access mechanism is designed to defend this problem in IEEE 802.11n mesh networks that supports high data rates. The performance of the proposed scheme is evaluated in a practical 802.11n indoor mesh testbed, that shows a significant performance improvement compared to the standard access mechanisms. © 2014 IEEE.