Firstly, let me say that I've played virtually every tennis game there is, all the Virtua Tennis games, all Top Spin 1, 2 and 3, and all the Dream Match Tennis games.
In comparison, the direction control in Full Ace is the most difficult to control, it is far too sensitive. Even tapping a direction at the point of contact sends the ball out or long, even from the tightest of angles or farthest distance from the baseline. There is no feel for the duration for which a direction button needs to be pressed and the angle of the shot; it's rather difficult to control or impart any angle to a shot by just tapping the direction at the last moment.