Had he devoted himself to pitching baseballs, he may have been wearing a college jersey and throwing fastballs during the summer of 1991, as his father had done years earlier. Instead, Jim Courier decided at age 13 to put down the bat and glove and go with tennis. Courier trained for seven years, often doubting his own ability. But his hard work paid off in May 1991 when he defeated his one-time camp roommate, Andre Agassi, at the French Open in Paris to win his first Grand Slam title. (The "Grand Slam" is made up of four tournaments: the Australian Open, French Open, U.S. Open, and Wimbledon in London, England.)

Over the next three years he won three more Grand Slam titles and in 1992 became the first American male since John McEnroe in 1985 to finish the year ranked No. 1 in the world. Over the next few years Courier struggled to...