Challenge System (Line Calls)

When electronic line-calling technology is available, a player who disagrees with an "in" or "out" call can challenge it, triggering a review that shows the tracked trajectory and landing point of the ball, typically displayed on a stadium screen or broadcast graphic for both players and spectators to see. If the challenge overturns the original call, the point is replayed or awarded based on the corrected call and the player keeps their remaining challenges for that set; if the challenge is unsuccessful, the original call stands and the player uses up one of a limited number of challenges (traditionally three per set, plus one additional if a set reaches a tiebreak). Some tournaments have moved to fully automated electronic line calling with no human line judges and no challenge system at all, since every call is made by the tracking technology in real time.

The challenge system changed match strategy at the professional level in a small but real way — players now factor the state of their remaining challenges into decisions about whether a close call is worth disputing, since running out of challenges removes the safety net for a genuinely bad call later in the set. It also shifted a source of player-umpire conflict that was common before the technology existed, since a contested call can now be resolved definitively rather than through argument or umpire overrule alone.