In the small confines of the US Open outside courts, it is virtually impossible for players not to hear exactly what fans are saying, even when they are focusing on a potentially career-defining match.
Shouts from courtside are almost universally supportive - but some players are starting to believe all is not what it seems.
In a peculiar New York incident, Switzerland's Leandro Riedi reported a vocal fan to the chair umpire - for cheering him on.
Exasperated by losing a break, 23-year-old Riedi suspected the man was a bettor and decided to act.