Many tennis players rely on rhythm to be successful. Hitting several groundstrokes in a row often gives a player confidence to hit that more aggressive shot as the point gets longer.
There are quite a few ways to disrupt your opponent's rhythm. One is to try to keep the points as short as possible and not let your opponent hit too many balls. You can do this by serving big, serving and volleying, or focusing on and executing the serve plus one or return of serve plus one idea.
Another way to change up the rhythm is to vary the spin on your shots—hit one normal ball, one with extra topspin, and then one with slice, etc. You can also vary the speed that you hit the ball—slowing it down or speeding it up on different shots.
Be aware that by taking the rhythm away from your opponent, you may also be taking your own rhythm away. Try to get comfortable hitting different types of shots and dealing with the variety of shots that are coming back. You can always establish some rhythm at the beginning of the point and then change it up.
An alternative is to play a few points with rhythm and then make a conscious effort to vary the shots or shorten the point here and there. The ability to control the rhythm in the match can lead to frustration on your opponent's side of the court, and help put the chance for success more in your favor.