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A Knuckleball in baseball is a ball without spin where the interaction with seams and the air fluids result into very sporadic flight paths that can change during the flight. I have observed knuckleball-like shots in tennis and shots where the ball changes direction after hitting the round.
How do you hit a shot without spin in tennis and is there anything equivalent to a knuckleball?
There is no way to hit a "knuckle ball" type shot in tennis - on purpose, at least. I've seen it happen very rarely after someone hits the ball with their frame or some part of their racquet other than the strings but I've never known or heard of anyone hit that kind of shot off their strings. The very nature of putting the strings against the ball is going to cause the ball to spin a certain way. Pretty much every shot in tennis has some kind of spin on it - the serve, ground strokes (both topspin and slice), volleys, drop shots, etc. They all have spin. It is not really something to try and get good at anyway, because it would mean the easiest kind of shot possible for your opponent to return (a ball with no spin poses no challenges to the player trying to hit the ball, unlike a ball with a certain amount of spin/pace).
The shots where the ball changes direction after hitting the ground is when the player hits a slice and puts a sideways spin on the ball so that after it hits the ground, it will go in the direction of the spin. Most experienced players are able to tell what kind of spin was hit on the ball and can position themselves accordingly so they are standing in the right spot to return that kind of shot.
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.
