It is well known that the power transfer efficiency (PTE) of a wireless power transfer (WPT) system is maximized at a specific coupling coefficient under the fixed system parameters. For an adaptive WPT system, various attempts have been made to achieve the maximum PTE by changing the system parameters. Applying the input matching networks to the WPT system is one of the most popular implementation methods to change the source impedance and improve the PTE. In this paper, we derive the optimum source condition for the given load and the achievable maximum PTE under the optimum source condition in a closed-form. Furthermore, we propose a method to estimate the input impedance, without feedback information, and an input matching network structure that transforms the source impedance into the optimum source obtained from the estimated input impedance. The proposed technique is successfully implemented at a resonant frequency of 13.56MHz. The experimental results are in close agreement with the theoretical achievable maximum PTE and show that the use of only a single matching network can sufficiently achieve a PTE close to the ideal maximum PTE.