NEW YORK—Leylah Fernandez is a self-described “happy-go-lucky girl” having the time of her life at Flushing Meadows, raising her fists, pumping her arms, and riling up crowds while beating two past U.S. Open champions to reach her first Grand Slam quarterfinal. A day before her 19th birthday, the unseeded left-hander from Canada grabbed the last five games to eliminate 2016 title winner Angelique Kerber 4-6, 7-6 (5), 6-2 on Sunday, demonstrating that an earlier upset of defending champion Naomi Osaka was certainly no fluke. With grit and guile, plus a veteran’s poise in the face of big deficits against much more accomplished opponents, Fernandez is displaying strokes and a demeanor that left Kerber offering this assessment: “She can go really far in the next few years.” How about over the next week? There’s no time like the present for the teens in tennis: Also into the quarterfinals with a win …