Tennis matches regularly feature grunting, but that's supposed to come from the players as they hit the ball back and forth in a rally.
Soon enough, though, the noise became impossible to smile at and ignore.
It's unclear whether the noises were coming from nearby, or within the stadium.
"Well, that is the most weird situation", announcer Mike Cation said on the broadcast.
"Honest to god the most absurd, and hilariously amusing thing I've ever seen. um. heard", he added.
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"Nope, that's not a phone".
In the second best moment of the video, he makes a final comment about the interruption and then goes right back into talking about the match.
It turns out, it was a real couple in "an apartment across the lake". "At least somebody's having a good night".
Cation later saluted the responsible couple on Twitter, writing "Sounds like you guys had a good time!"
According to Deadspin, fellow player Matt Reid who was commentating on the game claimed Tiafoe's reaction was one of the funniest things he had ever heard.