Williams plays 2/3 sets whereas Federer/Djokovic/Nadal play 3/5 sets. That makes it easier for her to contest doubles and win more prizes, so I don't think the number of titles is as easy to compare. It also means spectators are watching men a lot more. So it makes sense for more prize money to be attributed to them and endorsement too. Why wouldn't you pay Federer more considering he will be seen on court for much more time than Williams?
But endorsements are a bit weird anyway. Why shouldn't good-looking, athletic-looking people not consider it unfair that sports-stars get the big endorsements?
Is it unfair if spectators would rather watch men than women?
Is it unfair if spectators would rather watch one sport than another?
Is it unfair that there is so much more prize money and endorsements than several decades ago?
But even assuming she is correct and she has been unfairly underpaid relative to some others, does it follow that she should have been paid more? Aren't there a lot of other simultaneously unfair relations which should reduce her pay? For example, isn't there an unfair lack of interest in some other sports? (Shouldn't tennis players have their income reduced to help out less popular sports?) Is the proportion of more going to the top players fair? (Isn't it arguable that the lower tiers of profession should be paid more at the expense of the winners?) Are all of the workers who support Williams paid fairly? (Not just her direct support staff, but everyone who works at tournaments, broadcasting matches, and in the production of goods which she endorses. Isn't arguable that some of the thousands of workers who sell tickets, serve food and drinks, maintain tennis courts, officiate matches, photograph and video should be paid more? What about those are paid very by Nike in comparison to her contract? (Apparently the average Nike retail employee earns $12 per hour and factory workers earn about $1 per hour)
So it seems like her argument is that unfairness relative to her matters, whereas other forms of unfairness do not matter. Therefore it seems she is simply advocating for her own self-interest rather than really making an argument about unfairness.