Mertens, 22, has climbed into the top 20 after winning tournaments in Lugano, Switzerland, and Rabat, Morocco, during this clay court season. She won 13 straight matches on clay before losing to top-ranked Simona Halep in Madrid. In January at the Australian Open, Mertens became only the third Belgian woman to reach a Grand Slam semifinal. She upset Elina Svitolina in the quarterfinals before losing to the eventual champion Caroline Wozniacki in two close sets. While her two-handed backhand is her best shot, Mertens wins matches with her high tennis IQ, playing the right shot nearly every time. She uses topspin loops, slices, and drop shots when she’s defending, and she has a knack for keeping her opponent on the run and hitting balls out of the strike zone.