As to whether it alters the flight, I dont know, but it is there to stop the ball falling apart, as it crosses the equator 4 times.

With cheaper tennis balls, they tend to fall apart easier, as the wiggly line is not as thick, thus cheaper.

Essentially tennis balls are hollow spheres of rubbery material pressurised with air, and covered with a furry fabric. Why the elaborate construction? Partly because it gives a good ball with satisfactory behaviour, and partly because that design has become traditional. Apparently the rules do not demand any seam, and stitched seams are forbidden. The solid material in the seam is the exposed part of the vulcanising adhesive used to attach the outer fabric. Nowadays the fabric probably could be replaced by seamless felting, but somehow, after more than a century, a seamless tennis ball seems wrong, doesn’t it? I suppose that the seam must have some aerodynamic effect, but only slight, and because the seam is so symmetrical, it also should be largely neutral.

Apart from sentiment, why that particular seam pattern? It is desirable that any seam should be symmetrical and undistorted, but the fabric is cut from flat sheets and it is difficult to cover a sphere with a single, strong, symmetrical piece of flat fabric without troublesome distortion. As a compromise two infinity-shaped pieces of fabric can conform to a sphere, leaving an elegant 3D Yin-Yang seam between them. Personally I could hardly imagine any solution more aesthetically pleasing and practical.

Mind you, how would you feel about a two-tone tennis ball, one for Yin and one for Yang?

Cheers,

Jon

In addition, a visible seam on the allows players to more easily judge the spin of an approaching ball. Having a wiggly seam is better for this than a straight, as visual cues are available even if the ball is spinning along the seam's axis.

In some cases I expect it helps the line judge when calling "OUT".

The 'wiggly lines' will make a more distinctive outline in the chalk when it hits the line.