Modim Anachnu Lach

During Shemoneh Esrei we say "Modim Anachnu Lach". Why do we say the feminine "Lach" instead of the masculine "Lecha"? Are we thanking a feminine aspect of Hashem, e.g., the Shehinah? If we are thanking a feminine aspect of Hashem, why are we thanking a feminine aspect of Hashem instead of a masculine aspect.

In biblical Hebrew the feminine Lach is used for the masculine form as well in hundreds of cases. (Gen. 3;18 4;6,12 14;21,23 15;1 just to mention a few)