I don't only use the TV ratings as a measure of popularity.
I realize that this is just my experience, but among the people I know who do not follow tennis, not one of them knows who Sharapova is. But, they do know who Serena, Venus, and Clijsters are, and some of them know who Stosur is. Apparently you have to either have a baby or beat Serena to be known amongst the masses here. I do know one guy who knows who Sharapova is, but that is only because he saw her in a camera commercial, and he only paid attention to the commercial because there was a talking dog in it.
I have asked all of my tennis friends if their non-tennis friends know who any tennis players are, and they have had the same experience I have - their friends know Serena and Venus, and Clijsters because such a deal was made over her having a baby. I think some people over-estimate the ability of Americans to pay attention to anything for more than 5 minutes. If Sharapova was on some singing-for-humiliation show, or if she was on some crappy reality show alongside the Kardashians, people might know who she is.
You mentioned Nadal and Djokovic - yes, they are popular among tennis fans, but again, people who do not follow tennis so not know who they are. Regardless, the discussion regarding Sharapova is different, as a few people here have been arging that she has reached a level of celebrity that has made her well-known across the entire world, while Djokovic and Nadal have not become celebrities at the level of Sharapova. That is what I am talking about when I talk about popularity - not the popularity of a tennis player among fans, but the popularity of a tennis player as a celebrity, and among people who are not tennis fans. Seriously, if Sharapova were well-known in the U.S., one of the crappy reality show channels would have offered her a tv show by now.
First of all, to say Stosur is more popular than Maria in the US is just crazy talk. Second of all, I guess we've had different experiences because most people I talk to know who Maria is. They usually also know who Serena and Venus are(though not always by name) but I've rarely seen someone knowing who Serena and Venus are but not Masha.
When I went to the US Open two years ago people went CRAZY when Maria was practicing. They would hide in bushes just to see her. Lots of people also went to an outer court and stood on the highest row that looks over the practice courts to see her practice. We actually got kicked out of the court because so many people came just to see Maria practice.