mutter to themselves, of course

Tennis is the sport in which you talk to yourself. No athletes talk to themselves like tennis players. Pitchers, golfers, goalkeepers, they mutter to themselves, of course, but tennis players talk to themselves - and answer. ... Why? Because tennis is so damned lonely. Only boxers can understand the loneliness of tennis players - and yet boxers have their corner men and managers. Even a boxer's opponent provides a kind of companionship, someone he can grapple with and grunt at. ... People sometimes mention the track-and-field runner as a comparably lonely figure, but I have to laugh. At least the runner can feel and smell his opponents. They're inches away. In tennis you're on an island. Of all the games men and women play, tennis is the closest to solitary confinement, which inevitably leads to self-talk.

-- Open by Andre Agassi