In dense wireless local area networks (WLANs), the primary causes of interference and hindrance to spatial reuse are the well-known hidden node problem (HNP) and the exposed node problem (ENP). In this paper, we propose a joint solution to these problems. The proposed mechanism, referred to as Probe/PreAck (PR/PA), utilizes the concept of a two-way handshake for efficient channel reservation and spatial reuse by means of two control frames called PR and PA. The PR frame is designed for the semi-reservation of a channel initiated by a transmitting node, while the PA frame is used for the receiver-oriented permission of a channel reservation. Once a transmitting node advertises a PR frame, the channel is temporarily reserved, and the channel reservation is finally completed after the node receives the corresponding PA frame. Otherwise, the channel reservation is immediately released. Unlike the ready-tosend/clear-to-send mechanism, which is a well-known solution to HNP, the proposed mechanism does not unnecessarily prevent nodes from accessing the channel but selectively blocks transmitting and receiving nodes when they overhear the PA and PR frames transmitted by neighboring nodes, respectively. In this way, the proposed PR/PA mechanism effectively deals with both HNP and ENP in a unified framework. We further enhance the PR/PA mechanism by devising an immediate destination switching scheme, which is implemented in access points (APs) to improve the downlink throughput. If an AP fails to complete the exchange of PR and PA frames with a specific destination, it sends another PR frame to a different destination node without performing a new back-off procedure. Moreover, we adopt the transmission time control scheme to assure successful spatial reuse in multiple basic service set (BSS) WLANs. By adjusting the transmission time of the data frames simultaneously transmitted in different BSSs, severe interference between the data and acknowledgment frames can be avoided. The results of a simulation study confirmed that the proposed mechanism outperformed conventional mechanisms in dense multi-BSS WLANs; the downlink throughput was increased by more than 10 times while the overall network throughput was increased by approximately 50%.