While there are some valid points here regarding the link between an off the wall mother and father in the role of a successful athlete, there are two simple reasons American tennis players currently troll the lower rankings. And unruly parents do not figure into either one.
First: Have you checked the rate of childhood obesity in this country lately? It's beyond embarrassing and ought to be the next major "cause celebre" in America. Half the women in the entire USTA run the baseline sporting muffin tops, nary a care in the world about the spillage over their way too small skorts. Instigate a national fitness program a la Gerald Ford's Presidential Physical Fitness Program in the '70s, and you'll see many more champions ten years from now.
Second: Can someone puh-leez teach these kids how to serve and volley?! John McEnroe brings this up time and time again, and it obviously falls on deaf ears. You simply cannot convince a kid in this day and age that beating a tennis ball in the hot sun for 5-6 hours in hopes of a one point victory is the way to fame and fortune. However, if you show them that hey, check this out! You hit a 120mph serve, run to the net, hit another shot and, boom! Opponent submission. This is such a basic tennis skill, yet no one does it, making the game unwatchable. Andy Roddick not attempting a volley at least every third point is like Tiger Woods foregoing his short game every third hole. Shorten the points, shorten the sets, and shorten the matches- a simple recipe for American youth to start cooking once again.