There was a recent review of where the balls actually landed by the male pros, including Djokovic.
Because of their "heavy ball" as r2473 explained above, most of their shots did just land past the service line, or midway between the service line and baseline.
It just emphasizes that not even the pros try to hit that incredibly close to the baseline on most shots.
Obviously there will be times to hit deeper like on an approach shot down the line - in part the higher net height and no cross court angle will mean this ball will land deeper.
Many develop an ability to really crack a course shot deeper than their usual rally ball when they get the right ball to hit - right in their strike zone.
By cross court, I include inside out forehands.