Capriati trained with Macci until age 13 when she began attending the Hopman Tennis Academy at Saddle-brook resort in Wesley Chapel, Florida. Tom Gullickson became her next coach. By this time Capriati was already garnering a lot of attention in the tennis world. In 1988, at age 12, she won the United States Hard Court and Clay Court junior titles for ages 18 and under. The following year she won the singles junior titles at the French Open and the U.S. Open and the doubles junior titles at Wimbledon and the U.S. Open with Meredith McGrath.
At age 13 Capriati was already talking about turning professional. However, the Women's Tennis Association (WTA) rules did not allow girls to play in professional tournaments until the month of their fourteenth birthday. Even before her first professional match, Capriati had already signed a multimillion-dollar contract with Diadora, an Italian shoe and sportswear company. Capriati made her professional debut just before her fourteenth birthday at the Virginia Slims tournament in Boca Raton, Florida in March of 1990. Although she did not win the tournament, she became the youngest player ever to reach a professional final. She lost to the number three player in the world, Gabriela Sabatini.
At five feet seven inches and 130 pounds, Capriati was a teenage sensation who was capable of beating women who were older and more experienced than she was. She debuted on the WTA rankings at number 25. "Jennifer's strengths as a player are her aggressiveness, her unpredictability and her power," wrote Charles Leerhsen and Todd Barre of Newsweek in May of 1990. "Her groundstrokes are, as Billie Jean King says, too hot for many women to handle; her backhand is superior to [Steffi] Graf's."