6 min read EARLIER THIS YEAR , at the Australian Open, tennis superstar Jannik Sinner smashed ball after ball past an opponent on Margaret Court Arena. It looked so easy, as if he were playing a poor schmuck from the rec courts. The other guy was ranked No. 22 in the world. I sat in the stands. Everyone looked a bit bored. Sinner looked bored, too, but he always carries a kind of focused-bored demeanor. Commentators like to talk about the violence of Sinner’s ball-striking, as if that were the main key to his world No. 2 ranking. They miss one of the real secrets of his performance: the way he’s achieved his near-constant state of chill. He’s part of a growing cohort of elite athletes, including F1 master Charles Leclerc and skiing GOAT Mikaela Shiffrin, who are using brain training routines to achieve a state of engineered calm. They’re spending time at mental gyms that feature complex gadgets and data analysis, learning practices that build what’s known as mental economy.…