Seeing a player launch into an on-court tirade is nothing unusual, but Riedi shouting at someone in his native language turned out to be different.
The incident, which was first spotted and reported by Bounces, external, came in a pivotal moment during his second-round win over Argentine 19th seed Francisco Cerundolo.
Riedi had lost the first two sets before winning the third, but had just given up a break he had earned earlier in the fourth.
One particular man – who Riedi had also spotted during his first-round victory against Spain’s Pedro Martinez – was stood in the corner of the court and bore the brunt of his frustration.
“In that moment he was saying the wrong words to me,” Riedi, who has faced online abuse from gamblers, told BBC Sport.
“He was trying to be positive but for me it was like ‘be quiet, you have nothing to do with me’, you’re just betting on me.
“He was not a Swiss guy and he was always on his phone. So for me… he’s probably a bettor.
“In that moment I just didn’t want to see or hear this guy.”
Following his outburst, Riedi told chair umpire Scotty Moore about his suspicions.
Riedi claims both Martinez and Cerundolo – who could not be contacted by BBC Sport – also thought the fan was a bettor.
The man left the court by “his own volition”, according to the United States Tennis Association (USTA), and was not escorted out by security.
“Maybe I’m wrong, but I feel I’m correct because I’ve experienced it enough – that he was betting,” said Riedi.
“If he really is a supporter and he didn’t bet then I’m truly sorry, but I have a gut feeling he was a bettor.
“I told the umpire if I lose, this guy will text and say ‘I hope your mother dies’. This has happened too many times.”
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