Sprinting is an easy one. If you don't have the right muscle fibers, you'll never be fast relative to a good sprinter, period. Training gets sprinters those last bits of speed that are critical to win, but the necessary predicate is the right genetics. Likewise marathoning -- it's not Kenyans who are great marathoners, it is people from one specific genetic background who are well-suited to long distance running.
Center in the NBA requires more skill acquisition, but I can still confidently state that there will not be a lot of centers under 6' in the NBA. The position puts a huge priority on genes and good nutrition together working out as height.
Bartoli, no offense to her, isn't particularly physically gifted. One of the great things about tennis is its accessibility due to the priority it accords practice. A small or slow man, say, may not have the height or foot speed to win Wimbledon, but can still become a highly skilled singles player, who can easily donut a much bigger, stronger, more athletic 3.0 . Take a good long distance runner and have them run the 200 against an 19 year old who was a good high school sprinter, and barring injuries or other types of lightning striking, the long distance runner simply will never have the physical material to compete at sprints.
For skill sports, like tennis or skiing or golf, it is relevant in that people who want to lift their skill level, can. Because these sports prioritize practice, people are not nearly so bounded by genetic gifts as they are for other sports. It is then relevant in that the way to get better is to practice in a way that develops skill. For golf, say, driving range is much more important than going to the gym. For tennis, hitting on a wall (or ideally with a partner) and practicing serves is much more important than the gym. Etc.
There is a lot here but one thing that stands out is the statement that Bartoli isn't particularly gifted. How do you know? By watching her play you have come to this conclusion? I guess you just want to ignore her reaching a rank of 7 in the world. We are talking about the elite of the elite, not some club pro. There are 2900 women playing NCAA D1 tennis. I guess with a some practice they could all break the top 10?
Your last paragraph makes zero sense for elite levels. Once again - every sport prioritizes practice whether it is the actual sport specific drills or in a gym. Just because people who play tennis or golf look to be in the normal size range does not mean their genetic makeup is not exceptional for the sport. A weekend warrior could improve with more practice, but that would not make the WW able to compete on an elite level.
The top 50 athletes in any sport have pretty much the same access to practice, training, diet/health maintenance and equipment. How come most of them never reach #1? The answer is genetics.