NEW YORK—Unseeded British teenager Emma Raducanu went from qualifier to champion at the U.S. Open in just her second appearance at a Grand Slam tournament. Raducanu beat Canadian teenager Leylah Fernandez 6–4, 6–3 in the final at Flushing Meadows on Saturday to become the first player to go preliminary qualifying rounds all the way to a major title in the professional era. The 18-year-old Raducanu, who is ranked 150th, wound up winning all 20 sets she played in New York—six in qualifying, 14 in the main draw—and is the first woman to win the singles championship without dropping a set since Serena Williams in 2014. This was the first major final between two teens since Williams, 17, beat Martina Hingis, 18, at the 1999 U.S. Open and the first between two unseeded women. Raducanu broke to go up 4–2 in the second set, held for 5–2 and twice was a …