Some players confuse warm up and practice. A player should make a special effort to hit his shots directly to his opponent. Take all your warm-up serves before the first serve of the match. Courtesy dictates that you not practice your service return when your opponent practices his serve. A player calls all shots landing on, or aimed at, his side of the net.
Opponent gets benefit of doubt.
When a match is played without officials, the players are responsible for making decisions, particularly for line calls. There is a subtle difference between player decisions and those of an on-court official. An official impartially resolves a problem involving a call, whereas a player is guided by the unwritten law that any doubt must be resolved in favor of his opponent. A player in attempting to be scrupulously honest online calls frequently will find himself keeping a ball in play that might have been out or that he discovers too late was out. Even so, the game is much better played this way.