The Chair Umpire is much more than just the person who sits in a high chair and announces the score. They are the guardians of the Rules of Tennis and enforce them to ensure a match is played in a spirit of fair play.
Umpires are impartial and cannot be prejudiced. The Chair Umpire has the final word on all issues relating to on-court facts, for example whether a ball is “in” or “out”, if a player has a foot fault, a service touches the net, etc.
In matches where line umpires are assigned, the chair umpire has the duty to “over-rule” any clear mistake made by a line umpire.
In matches where electronic review (commonly called “hawk eye”) is in use, this can over-rule a Line Umpire’s and/or Chair Umpire’s decision (following a challenge) and cannot be appealed.
The Chair Umpire’s duty is also to control the match and enforce the Code of Conduct whenever required.
Team work is the key to being a successful Line Umpire. Not only do they call all shots related to their assigned line, they also work together with the Chair Umpire so that the match is played in a professional and fair manner.
Line Umpires are responsible for reporting any misconduct from a player that was not witnessed by the Chair Umpire. They also escort players on toilet breaks to make sure there is no breach of any rule.
Being a Chair Umpire or Line Umpire requires excellent eyesight and concentration. You need to be on top of any situation that may happen and always act in a calm manner.
Further detailed information is outlined in the 'ITF Duties and Procedures for Officials', which is available as a PDF below.