In this paper, an optimal resource allocation scheme for simultaneous wireless information and power transfer (SWIPT) systems is presented. The scheme considers nonlinear energy harvesting (EH) point-to-point single-input single-output (SISO) SWIPT system where the proposed scheme is based on timeslot switching (TS) aiming at maximizing the average achievable rate. The problem is formulated as a nonconvex optimization problem due to the presence of TS ratio binary aspect. We have solved the optimization problem by considering both the time-sharing strong duality theorem and Lagrange dual method. The derivation of optimal solution shows that the EH should be done in region of intermediate signal-to-noise ratio (SNR) while information decoding (ID) should be done in either the region of lesser SNR or higher SNR. Simulations are carried out in comparison with traditional state-in-the-art TS resource allocation schemes to evaluate the energy efficiency of the system in terms of transmission power, distance between source and destination, pathloss exponent, and minimum required harvested energy. Simulations showed that the proposed scheme improves energy efficiency with respect of different transmission power by 20%, 10%, and 3% for high SNR, medium SNR, and low SNR regions, respectively. Improvement with respect to other system performance metrics has also been noted for the proposed scheme.