NEW YORK—Emma Raducanu and Leylah Fernandez are so similar in so many ways: They possess enviable quickness and anticipation. They take balls low to the ground and redirect them with ease. They don’t care how much better-known or more successful opponents are. They love the big moment. There’s more. They’re both teenagers. They’re both unseeded at the U.S. Open. They’re both getting loud backing from the crowds. And now, remarkably, they’re both Grand Slam finalists. Raducanu, an 18-year-old from Britain who is ranked 150th, and Fernandez, a 19-year-old from Canada who is ranked 73rd, took wildly different paths to the championship match at Flushing Meadows on Thursday night. They’ll be back in Arthur Ashe Stadium on Saturday for the first major final between two teens since the 1999 U.S. Open, when Serena Williams, 17, defeated Martina Hingis, 18. Raducanu became the first qualifier to reach a Grand Slam final in …