No one loves a silly high school prank like Jimmy Fallon (and maybe that guy from school who still talks a little too much about the glory days). Whether or not you're of the comedian, this prank is entirely about the willingness of tennis stars to do something absurd on their own time.
Fallon asked athletes at the US Open to sneak ridiculous phrases written by The Tonight Show into actual interviews they gave during the tournament. A surprising number of them actually did it. The show collected the clips and strung them together to air on The Tonight Show. No one but the players knew this was going on, Fallon says in the segment's intro. It was a little bit to explain, but it's clear from Fallon's joy at the start of the segment that this went exceptionally well.
Now, it'd be pretty easy for the players to just drop in a goofy word here or there, but what they were asked to do was much harder. For instance, Andy Murray worked in, "I used to take cat naps, but now I take kitten naps. Half as long, twice as cute."
It's amazing how many players joined in and managed to keep a straight face while saying things like, "Game set match. Do the cabbage patch."