Edit - the more I review this list...just LOL
Tennis, gymnastics? Wtf
If you ever meet an elite college wrestler, SEAL, MMA, etc. you will know in 2 minutes of casual conversation that their intensity level is not normal. These guys are crazy intense all the time.
I'm not talking about some chubby JV kid on your wrestling team, but the real guys competing for HS state champ, college, Olympic. These are some of the toughest men on the planet. Most elite tennis players will readily admit they work hard but wrestling is just a tougher sport.
You can legally squeeze the krap out of someone, slam them to the mat, crossface their nose off, slap them (called a head tap), kick them (called leg sweep), rub the hair off their head, dig my forehead into their temple, dig your chin into their bag, hit their ball (high crotch), etc, etc. Just look at a wrestlers face sometime and it will be all cuts, black eyes, etc. Wrestlers are so mentally tough that they any physical pain is unnoticed.
On top of extreme conditioning, you need to remember the mental toughness to make weight. You're talking about not eat for 3 days a week, sweating off 5+ lbs, and extremely limited fluid intake. Extreme fasting takes mental toughness unlike other sports. I spent my wrestling career Angry at everthing and everyone. We worked our asses off, were starved, and essentially spent our live in pain.
My coach is in the hall of fame and is psychotic. He encouraged everyone fist fight each other like a pack of animals. He loved us fighting out of school. He loved fighting other coaches. He loved cussing at us and and inflicting pain in creative ways. Read up on J Robinson or Dan Gable. They are heroes but Gable has a clear personality disorder.
I'm not sure how you define "tough" as all elite athletes train hard but we had countless football studs quit the wrestling team. And the ones who stayed were the toughest and known as wrestlers. In HS I could take the "toughest" gymnast/tennis/etc and make him quit in a few minutes, and I did it regularly. The ones who made it through the first day
For example a cage fighting pit bull, a competition frisbee dog, and a racing greyhound all train hard. But the pit bull being forced to fight to the death on the reg requires a diff level of intensity, toughness, and training. IMO, a champion pitbull is prob mentally unstable like I think most champion boxers, MMA, wrestlers, etc.
Tennis. I love tennis but that's like comparing golf to MMA. Comical!