The area between the baseline and the service line, essentially the back half of each player's side. Where baseline players live. In modern tennis, heavy topspin has pushed many players even further back, standing several feet behind the baseline. Djokovic's ability to defend from deep in the backcourt and then suddenly take the ball early is what makes him impossible to outlast.
"She's playing from deep in the backcourt, retrieving everything, waiting for a short ball to attack."