Tennis etiquette in the modern day is a set of understood "rules" or "guidelines" that help ensure practical courtesy during every match played. It does not only benefit the players in the match, but it can also help spectators to not wreck the matches of those players. Since many amateur tennis matches are self-officiated, etiquette is there to make the game more efficient and enjoyable for all players.
Don't make your way onto a court unless it is open to the public and no one is currently on that court, or you have that court booked for that time.
When deciding who goes first the players should toss a coin or spin a racquet, whichever is agreed upon beforehand. The player with the first serve has an advantage, so leaving the order up to chance can even the playing field.
Keep your warm-up brief. Professionals keep their warm-ups to five minutes, and so should you!
Before serving, check to make sure your opponent is ready to receive the serve. Don't serve at your opponent's back. If your opponent hits a first-serve fault, do not return the ball unless it is too close to call.
Do not walk behind other courts while a point is in play. Do not interrupt a player's point to retrieve a ball. Do not cross another's court in the middle of a game.
Make your calls on the line quickly and clearly, but always give your opponent the benefit of the doubt if you are unsure. It can be rude to question your opponent's line calls; however, if you are really uncertain, ask once about the call before proceeding with the match.
Always make sure whoever is serving already has two balls on their side of the court. To feed balls, gently hit or roll them within reach of the server.
Stay attentive on the court during the match. Do not chat with spectators or check your phone mid-match. Do not attempt to distract your opponent either.
Firmly shake your opponent's hand and congratulate them at the end of a match regardless of the outcome. Have respect for your opponent and the game you just finished.