Decisions of whether a tennis ball had landed within or out of the bounds is made by a line umpire or a chair umpire. There are nine line umpires apart from the chair umpire who are positioned in a way that the lines which they are supposed to monitor lie exactly in front of their vision. In 1970s electronic line calling was introduced in order to increase the efficiency of umpiring in tennis. Grant-Nicks, Cyclops, Hawkeye, were some of the technological devices used in order to sense the ball’s pitch whether it is in or out.
With the advent of these technological advances, players could challenge the decision of the umpires if they had any doubts on the call made. Player receive three challenges per set. If they win the challenge, then they get to keep the challenge. Three lost challenges means a player can no longer challenge a call made by an umpire in that particular set. If the set goes to a tiebreak, both players get an additional challenge. If a player does not utilize all three challenges in one set, then the unused challenges are not carried over to another set.
A player is supposed to challenge a disputed call immediately. If a player takes longer to challenge a call, then the chair umpire has the authority to disallow the player from taking the challenge. In 2021, all line calls were made by Hawkeye Electronic Live Line Calling system. This meant that players could no longer challenge the calls. This system has not been implemented across all professional tennis tournaments, but is soon to make line umpires out of business.