Tennis in America has different levels. There are places where it is more popular than others, places where a lot of people do it. Other places, it is a class thing. Where I live currently in Pennsylvania, it is mainly a rich kid thing. Many kids believe it is too expensive and spend their efforts on other sports like soccer, basketball, football and baseball. Part of the reason for this is, in the winter there is no public place to play tennis in the winter. The Tennis & Racquet Clubs have very high membership fees. More public tennis centers are needed and more efforts need to made to draw kids to the game.
As for the interest in the pro tours, it varies. Many of my friends are tennis fans, but then again I've met many of them while working the US Open the past six years. There are a lot people who only know a few players and if they aren't playing, they don't watch. Basically, many tennis fans in America are casual tennis fans. ESPN2 has aired more tennis at the Australian Open so far this year which is good. More coverage is a good thing, but the American broadcasters really need to stop the total American bias.
As for players, right now the future of American tennis isn't that bright. KirilenkoRulz is right when he says that there really isn't any young player with top 20 potential. Alexa Glatch made it to the US Open junior girls final last year, but I don't see a lot of room for growth i her game. None of the other juniors have made much of a dent. I know junior tennis success isn't necesarrily an indicator of success as a pro, but most successful pros were at least in contention as a junior. I always watch the junior girls tournament at the US Open and I see a lot of potential future Top 20 players from Europe. But there aren't any players like Viki Azarenka, Agnes Szavay, Michaella Krajicek, Dominika Cibulkova, Tamira Paszek in the American ranks.