Serena and Venus were clearly phenomenal pure athletes. Serena is arguably the strongest womens player ever.
The Williams dad basically coached Venus and Serena. Think about it, he's not even a professional coach. Think about all the thousands of privileged tennis players that get unlimited professional coaching, facilities, Boletteri academy, etc.
I will grant you that tennis is a sport where overwhelming athleticism is not the primary attribute. But right after hand-eye coordination comes athleticism, and after that is top-tier endurance.
There is no shortage of athletes in high-revenue sports that will literally run through walls (see: rugby, american football) to succeed. And the vast majority of them don't. Look at Division I college sports: full of totally dedicated elite athletes receiving top level coaching. How many of them move onto the lower tiers of pro tennis?
Don't be fooled by the lottery winners / survivor bias of the top players. Yes they worked hard to get where they were. Yes they may work harder or smarter than some of their near-peers with equally flaggergasting natural talents. Yes it fits into a popular American narrative of hard work == success.
But look at Nick Krygios. I don't follow the guy's training, but it's pretty clear that he is as apathetic a top player as there is. Every tennis observer knows he has the physical gifts of an all-time great, almost naturally given.
Donovan Bailey was a Canadian sprinter that won the 100m in the Olympics in the 90s (no, not the steroid guy Ben Johnson). He was basically not training and goofing off a year before the Olympics and someone finally got through to him to actually train for it and got him focused, and boom a year later he was Olympic champ in the most selective, widely contested, pure athletic event in existence.
A lot of my frustration with the youth coaching sports pipeline is that they tell the parents to focus on one sport to exclusion. There is a racket where the high school teams will only have players coached by the coach in the offseason, "paid his dues".
So kids don't get a wide variety of sports exposure and don't have fun. Which is dumb. Sure there are these youth sports coach vampires making a buck off of the hopes and dreams of the parents, but it comes down to the idiot parents not realizing how things really work.