In all sports, a coach or instructor is necessary to help train, direct, instruct, and mentor their player(s) or team. A tennis instructor focuses on the individual, since tennis is typically a one-on-one sport. In general, tennis instructors work to help improve a player's overall game.
Similar to any other sport, there are many different levels of tennis. For beginners, instructors focus on teaching their player's the basic fundamentals of the sport. They refine swings, improve serves, and enhance basic skills and footwork. These instructors typically have years of amateur tennis experience.
In contrast, there are "top level" instructors who work to coach and mentor players who compete at a professional level. Tennis instructors study and evaluate a player's game to determine both strengths and weaknesses. Once an instructor understands what needs to be worked on, they can focus on the areas that need improvement and require more practice.
Professional tennis instructors also serve as mentors to their players, and they have to work on their psychological skills and techniques, as well. They help players manage stress, learn to keep focus in the heat of a match, and deal with the public eye.
Before beginning instruction, each tennis instructor must be officially certified. In the United States, The Professional Tennis Registry (PTR) and The United States Professional Tennis Association (UPTSA) offer instructor certification. There is no minimum experience requirement for instructors to have, but an instructor should generally have several years of tennis experience to become certified.
Every professional tennis player has their own instructor and coach, so there are not typically instructors who can be credited with the success of many great players. Ivan Ljubicic is Roger Federer's coach, and is considered one of the best coaches because of Federer's success. Patrick Mouratoglou is Serena Williams' coach, and she credits a portion of her success to his coaching.