We present sensor-DMAC (S-DMAC), a new mechanism for topology control in wireless sensor networks. A (connected) fraction of the network nodes is efficiently selected to perform the network operation while all other nodes are switched to an energy-conserving "sleep mode." Based on DMAC, a clustering protocol enhanced with backbone formation, S-DMAC reduces to a minimum the overhead associated with node selection, backbone formation and maintenance, thus increasing the overall network lifetime. Ns2-based simulations have been performed to compare S-DMAC with DMAC and also with GAF, a recent solution for topology control in wireless sensor networks. These results show S-DMAC as more effective in providing connected and energy efficient routes of data from the sensor nodes to the network collection point (sink).