NEW YORK (REUTERS) - Ashleigh Barty, the No. 1 seed at the US Open, watched on match point as her opponent's backhand sailed long on Thursday (Sept 2), prompting the familiar sound of a linesman yelling, "Out!"

There was no line judge, however. The call was a recording, triggered after an advanced system of cameras known as "Hawk-Eye Live" tracked the ball until it landed out of bounds.