We present a study of $D rightarrow K, l u$ semileptonic decays on the lattice which employs the HISQ action for both the charm and the light quarks. We work with MILC unquenched $N_f = 2 + 1$ lattices and determine the scalar form factor $f_0(q^2)$
. This form factor is obtained from a scalar current matrix element that does not require any operator matching. We find $f^{D rightarrow K}_0(0) equiv f^{D rightarrow K}_+(0) = 0.747(19)$ in the chiral plus continuum limit and hereby improve the theory error on this quantity by a factor of $sim$4 compared to previous lattice determinations. Combining the new theory result with recent experimental measurements of the product $f^{D rightarrow K}_+(0) * |V_{cs}| $ from BaBar and CLEO-c leads to a very precise direct determination of the CKM matrix element $|V_{cs}| $, $|V_{cs}| = 0.961(11)(24)$, where the first error comes from experiment and the second is the lattice QCD theory error.
