You are totally wrong, if we talk about worldwide popularity.
USA is not anymore an indicator of tennis popularity like it was in the past.
I can't speak for the rest of the world, but tennis has never been very popular in the U.S., and you're right, it has fallen quite a bit in recent years. I can't imagine how much more unpopular it would be if not for Serena and Venus. I think the only current player that people here really know much about other than those two is Clijsters, and that's only because such a huge deal was made about her coming back after having a baby, and she is in commercials for Citizen watches.
The U.S. ratings show only how popular NBC is.
I really am not getting that comment. NBC is not a pay channel. You don't even have to subscribe to cable or satellite to have it. Everyone with a tv in the U.S. can pick up NBC. If someone was really a big Sharapova fan, they could have watched the FO final.