Like @jamauss said, if you see the ball change direction after it bounces, that is a result of a slice shot, when a player follows through under the ball to create backspin, and occasionally (especially on drop shots - slices intended to land close to the net and stay close to the net after they bounce) a player will put side spin on the ball as well as backspin. Although slices have similar effects to knuckleballs after they bounce, the player receiving the ball can see clearly when and how his opponent slices the ball, so there is almost never a surprise in the direction of the ball after it bounces.
The closest thing to a knuckleball in terms of having know spin would be a completely flat groundstroke. By swinging straight through the ball, you can create minimal spin. This is very uncommon because it is difficult to produce a ball with no spin at all, but, more practically, flat groundstrokes are very uncommon because flatter balls are easy for the opponent to hit. A shot with backspin will force the opponent to adjust to the strange bounce, and the ball will slow down after bouncing, forcing the opponent to create more power rather than redirecting the ball's pace back across the court. A topspin shot, on the other hand, will fall more quickly, allowing the player to hit the ball harder without fear that it will go long. When the ball bounces, it jumps up and back towards the baseline, leading to a difficult ball for the opponent. Flat balls are generally hit from a high contact point (so the ball will go down into the court rather than long) with large amounts of pace - the only reason to hit a flat ball is because you can create more pace, as no energy goes into spin.