My list would be:
Federer
Serena
Agassi
Sharapova
Sampras
Mac
If we are talking worldwide recognition based on name value and nothing else. People are likely to know those 5 people regardless of how they feel about them. Now, if you are under 25, that list might start to change.
It makes no sense not to have Nadal there. He has the biggest social media following in Tennis. In terms of objective numbers, Nadal is the most popular player in the world (male or female) and has been for years. These are hard numbers, not anecdotes about some of your neighbours not knowing who Nadal is.
The media have done an amazing job brainwashing people into believing Federer is more popular than Nadal. If he was, his social media numbers would reflect this. They don't.