NEW YORK—No one is more surprised than Emma Raducanu herself. The British teenager became the first qualifier to reach the U.S. Open semi-finals by beating Olympic champion Belinda Bencic in straight sets on Wednesday. “I didn’t expect to be here at all,” Raducanu told reporters. “I think my flights were booked at the end of qualifying, so it’s a nice problem to have.” The upbeat and smiling 18-year-old has made short work of her U.S. Open campaign, winning all 16 sets she has played from qualifying through to the semi-finals, proving lethal at the net and from the baseline. Raducanu is continuing her remarkable journey through the tournament without her parents in the stands, telling reporters they would need a waiver—which takes a couple weeks—in order to travel. Their influence is still felt even from afar, however, as she maintains an on-court composure far beyond her years on the biggest …