Alexander Zverev felt he had to react when he heard a fan use language from Adolf Hitler's Nazi regime during his U.S. Open match. This wasn't the type of heckling that players are prepared to face and trained to ignore. It highlighted a challenge faced by players and the U.S. Open itself as the event draws the biggest crowds in its history: making sure fans are engaged but not disrupting the tennis — and how to respond when they do. In the case of Zverev's match, the spectator was sitting in a crowded section close to the court, where many...