Coco Gauff, the groundbreaking 19-year-old taking the tennis world by storm, is the one to watch after her first Grand Slam win at the U.S. Open this past month. This makes her the first American teenager to win the U.S. Open since Serena Williams in 1999. And while you may believe the professional tennis season is over with the U.S. Open in the rearview and the Australian Open still three months away, it is not.