At the time, the controversial call was well-covered by sports media. ESPN described what happened in detail, explaining to laypeople that tennis doesn't allow for mid-match coaching. When Mouratoglou was issued the violation, Serena Williams became very defensive, even during the match, explaining that she doesn't cheat and always plays honorably (which she does). The signal from her coach wasn't meant to be off-court coaching. Regardless, the violation was upheld, Williams was rattled, and Osaka went on to win the match. After the match was over, Williams demanded an apology from the umpire. She was ultimately fined $17,000 for the violations. It was all very dramatic. 

Kuritzkes didn't know about the no-coaching rule in tennis prior to the 2018 Open, and could only put himself in Mouratoglou's shoes. In the screenwriter's mind, William's coach had something he desperately wanted to say to his player but wasn't able to. That tension, Kuritzkes figured, would make a great movie moment. He said: 

"It was this big, controversial call. [...] I'd never heard of this rule and it struck me as this very cinematic situation: you're alone on court in front of thousands of people and there's one other person in the stands who cares about this as much as you do, but you can't speak to them. I started to wonder, 'What if you needed to have an urgent, important conversation with this person? And what if it was about something really personal and dramatic beyond the game?'"

It took a lot of mulling over, but eventually, Kuritzkes found a way to synthesize that tennis court separation into a movie. What if there was a love triangle between the coach and the two players? Dramatic gold.