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"It can't be that good," an amused Tiafoe shouted from the court.
Video from the event shows a mother in the stands instructing her young son to cover his ears.
The sounds were clearly audible for a few seconds before quieting back down, but they returned about a minute later.
Announcer Mike Cation speculated the sounds were coming from an "adult video" playing on someone's phone in the stands, but he later revised his assessment.
"Much love to the couple with the windows open across the way! Sounds like you guys had a good time!" he tweeted.
Tiafoe also said he believed the sounds were coming from a live session rather than a video.
"Never seen something more real it was amazing," the player tweeted.
Tiafoe ended up beating Krueger 6-3, 6-2.