Really your logic is very strange. I am not saying it is poor. It is strange. And it is clear why you think that way, you simply try to go around the facts. The quantity of people who know the person indicates the person's popularity. The spikes you mentioned prove that more people know Sharapova than Nadal. Period. There is nothing more to discuss about.
Go look at September of 2009, and September 2010 for Wozniacki. Her peaks during those months is much higher than Sharapova's. So I guess during those months more people knew who Wozniacki was than Sharapova? If you want to keep considering those spikes as some evidence of how well known she is, you better be prepared to use the same argument regarding Wozniacki. My god, Wozniacki is a tennis icon, worshipped and beloved by all Americans!
And, once again, we are not talking about how many people know who Sharapova is as compared to Nadal or anyone else. Whether or not more Americans know who Sharapova is compared to Nadal, that is in no way evidence that she is an icon, known by people who don't even follow tennis. She might be popular compared to some other players but that doesn't make her well-known among non-tennis fans.
If 1,000 people know who Nadal is, and 2,000 know who Sharapova is (and let me be clear that I am not suggesting these are numbers are true, it is merely an illustration), when you consider the total population, they are both known by a very small percentage of the population. Sharapova being known by twice as many people as Nadal does not in any way reflect how well-known and loved she is.