The sex noises can be heard at around 15 seconds into the video above, and then again around the 1:50 mark. It's unclear whether the noises were coming from nearby, or within the stadium.
At one point during the match Tiafoe yelled, "It can't be that good."
Though commentator Mike Cation opined the sounds were coming from a nearby open window.
Much love to the couple with the windows open across the way! Sounds like you guys had a good time!@SarasotaOpen— Mike Cation (@MikeCTennis) April 19, 2017
Tiafoe won the match 6-3, 6-2.