Players can retire from tennis matches at two points; either before the match has started (and it is granted as a walkover for their opposition) or during the match.
Pre-Match Retirement
This occurs when a player sustained an injury and cannot compete in their next match.
In this case the umpire will log into the tablet and the match will process as follows:
A player can retire from injury at any point in a match. The packets are normally sent after a 'Point' packet mid-game, or during a changeover.
It important to note that a retirement packet may or may not be received after a 'Medical Timeout' - sometimes a player will request medical attention and attempt to continue before retiring, but in other circumstances they will be too injured to continue and will retire without treatment.