Type of string and tension are important for your game
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Tighter strings give more control as the ball tends to "Flatten" more on the string bed and create a flat surface to guide the ball as it is released from the racquet.
The tight string does tend to dissipate a lot more of the energy of the balls momentum into string bed deformation and thus reduces the power that can be generated by a certain stroke on the ball.
Tight strings are also a major cause for Tennis Elbow.
Looser strings generate more power because the string bed deflects and almost ejects the ball from the strings as the string bed rebounds. This however reduces your control as you are not so much "In charge" of when (or where) the ball is released.
Looser strings are fine if you are a player that uses long smooth strokes, imparts spin or slice on the ball, keeping the ball on the strings longer. This overcomes the effect of the soft string bed to some extent.
TIP:
I don't think anyone will deny that Pete Sampras was one of the greatest players of all time and what Pete did was to use a lighter head racquet, massive string tension and build back the power with side weighting. This gave him the power AND the control obtained with tighter strings and it surely worked. Pete also had the muscle to handle a slightly heavier mass in the head of the racquet. The added side weights also stabilises the racquet, prevents twist and enlarges the sweet spot left to right, quite a nice trick I use all the time and it is not widely known. Lead style tape can be obtained from any online sports shop, use about 2 inches on each side of the racquet and try it, if you want a little more place a further 2 inches of the inner surface next to the first strip.
STRING TYPES AND THICKNESS
Thinner string around 1.2 or 1,3 mm is best. Irradiated strings are best as the molecules are aligned so that string tension is not lost under play. Replace strings in two string racquet 2-3 times a year or in a three string racquet, don't even bother as the strings do not rub and notch and you change them more from a guilty feeling than a need.