AUCKLAND, New Zealand — U.S. Open champion Coco Gauff overcame hard-hitting Czech teenager Barbara Fruhvirtova 6-3, 6-0 and extended her winning start to the year and Auckland Classic defense to the quarterfinals on Thursday. Gauff immediately broke Fruhvirtova in the first game of the second set as she began to find more depth in her returns. Gauff had to save …