Players have become better and better at handling the deep, hard shots that most of the modern players use to get their opponent in trouble or to win the point.
Many of today's best players are capable of taking a few steps back and using the speed and depth of the shot to hit an even better shot back. Very few players are using the shorter shots to keep them honest and to force the opponent to cover a bigger area.
Try hitting some low, short balls in addition to your hard and deep shots—these do not necessarily have to be slice or drop shots but just a flatter ball that is lower over the net. This will force the opponent to cover the area in front of them and also make it more difficult for them to deal with the hard, deep ball.
Another great shot to use is the shot in between deep and short depth (ball bouncing near the service line) with some angle. This will pull the player forward and off the court and unless they get to the ball early and are able to use the angle, it should leave a wide open court for your next shot.
The use of the shorter balls will make it more difficult for your opponent to hang way back and it will allow you to use your hard deep shots more effectively.