A tire has a tread, with the tread having a plurality of ground engaging tread elements creating a tread surface and at least one of the elements has a sipe therein. In planes parallel to the tread surface, the configuration of a radially outer portion of the sipe is defined by a continuous non-linear curvature, the curvature having at least one radius R. The value of the radius R of the sipe curvature increases from the tread surface to a base of the sipe and the base of the sipe has a configuration defined by a radius R having a value of infinity.