Electronic line-calling system that tracks the ball's trajectory using multiple cameras. Players get three unsuccessful challenges per set (plus one more in a tiebreak). The slow-motion replay on the big screen is one of the most dramatic moments in modern tennis. The collective gasp when the ball is shown clipping the line by a millimeter? Unmatched.
"She challenges the call. The stadium goes quiet. Hawk-Eye shows the ball caught the line by two millimeters. Challenge successful."