[ArrichMer]

Seriously. The only way you can guarantee that a 4-year-old with a tennis racquet will go to the US Open is to buy them a ticket.

The single most important thing is that the child wants it. If they do, you'll know. From there you can seek reliable advice about what path to follow. But it scares me, really really scares me, to hear people ask questions like, "I'm going to give a 4-year-old a tennis racquet/baseball glove/ice skates/basketball for Christmas, how do I turn them into a successful professional athlete?"

To some extent, I agree. But it is also true, as politically incorrect it might sound, there have been many cases in which a kid got pushed by a parent to do well in some field, and it was only after growing up a bit that the kid started really getting attracted to it. The desire to succeed often starts out as a parent's dream and is then internalized by the child. Example: Williams sisters. I'm of course not advocating slave-driving, but making the kid (within reasonable extents) pursue a goal is not necessarily detrimental.