It all comes down to hair . The hair on a new tennis ball tends to be smoothed flat, while a ball that’s been knocked around a bit will be more fluffy. Tennis players may check three balls or more before serving so that they can select one smooth ball and one fluffy ball. The smooth ball is used for the first serve.
I have noticed on the slow speed clips when hit I can see an explosion of fibres from the ball.
The players most probably have an innate feel how fast and how it will respond to the playing surface by how much of the fibres are present or lost.
I heard a commentator talking about how they keep the bald balls and reintroduce them sometimes.
Which would be funny considering most of the guys have lovely smooth arms and legs.
I don’t think they can work anything out at that speed but does show how hard the ball gets hit.
Ouch.
The balls are temperature controlled for pressure consistency then they stuff them in a pocket.