Frances Tiafoe is ranked No. 17 in the world in men’s #tennis and has twice reached the semifinals at the U.S. Open, so it might be hard to imagine the 27-year-old American ever struggling with a racket—let alone this year. But that is exactly what happened when Tiafoe tried #padel for the first time this spring, while he was in Spain for the Madrid Open and as he continued to play every day for weeks, sometimes against the sport’s pros. “It’s actually pretty tough to be good at, even for former tennis players or current tennis players,” #Tiafoe recalls. “I was horrendous compared to them.” Yet the steep learning curve didn’t diminish his enthusiasm for the #sport, a hybrid between tennis and squash that is played in a glass-encased court and is growing quickly in Europe. And while padel (traditionally pronounced pah-DEL’ but often said like “paddle” in English-speaking countries) currently has only about 100,000 recreational players in the United States, Tiafoe believes the sport’s ...