NEW YORK -- Three years ago, a reporter asked Sloane Stephens to point out a younger tennis player most fans might not have heard of yet, someone she thought could become a household name some day.

FLUSHING MEADOWS, N.Y. — Coco Gauff’s U.S. Open ended just as she had warned it might. On an error-strewn afternoon of struggle on her forehand, the first shot she and everyone else learned as a child ...