Tennis spin has changed dramatically in the past 20 years:

...[T]he co-poly strings in use today -- which spread through the pro game only over the last decade or so -- generate more spin than ever. They do so because they're more slippery than prior string designs. Because the strings easily slide across one another, they can slip back and then snap back to position -- all while they're grabbing the ball -- to create more spin.

More details in the related article: "The New Physics of Tennis". (Via MR.)