“Genčić herself was no stranger to unorthodox training. As a child her father would sellotape tennis balls to the heels of her shoes so that she’d get used to walking around tip-toed.”
It turns out this was not Gencic. I have searched high and wide (is that a phrase?) to find where online  I originally got it from, but weirdly, it’s just not there. I swear I spotted it on a site. I genuinely remember the moment, but now it’s gone - or never existed. A manifestation of my sleep deprived 3am awake brain. I think I thought it conceivable given the weird training she provided to Djokovic, which outside of chasing the just bombed locations of Serbia, included reciting poetry to her, holding a trophy and declaring, “I’m Novak Djokovic. I won Wimbledon.” and, as reported in The New York Times: Gencic said she explained that one particular piece was like a tennis match: “You start slowly and then stronger, stronger, stronger,” she said.

Tchaikovsky’s “1812 Overture” left a particularly deep impact. “I could see he thought it was wonderful,” Genčić said. “And I explained to him, ‘When you play a match, Novak, and this is very important, when you play a match and suddenly you feel not very good, remember this music, remember how much adrenaline you have in your stomach and your body. Let this music push you to play stronger and stronger.’”
“He understood,” Genčić said. “He was 11 years old, but he understood.”
Anyway, this fact about the balls being taped to the feet is wrong. I’m sorry. It was not Genčić. It was tennis player Marion Bartoli whose dad did that. 
 

A small note that Agassi noticed that, when in the deuce court, Becker would slightly pop his tongue out of his mouth. If his tongue stayed straight, he was either serving up the middle or to the body. But if he put it to the side, he was going to serve out wide.