I'm sure it must be more popular in Spain than in the US. What gave you the impression it isn't?.
Tennis is very popular in South America. At least any big news - say slam finals - are mentioned in the front page of newspapers. Something like the winner of a masters will be on news on TV, when it was even rumoured Nadal was going to come here it was in the front page of newspapers.
When Marcelo Ríos would play a big match, you could literally hardnly find people on the streets, kinda like what happens when there's a big football match going on.
""If doesn't exist in sport. If never comes. You have to do it" Nadal