I hate to break it to everyone that is going to automatically call Nick Kyrgios a liar, but he's absolutely, 100% right in passionately denying that he swore.
I mean, I may read lips at a 4th grade level, but - since I already knew what "fuck" looked like heading into 3rd - I can definitively say that was indeed a "fuck". Unfortunately for all of you assholes tsk-tsking from under your high hats, a curse can't be recognized as voluntary without a target. Like, of course the most polarizing player in tennis doesn't believe that he used profanity. He's a jackass, not a psychopath. You know what type of people can recall every explicit conversation they have ever had with themselves purely out of frustration? The type of people you genuinely should be keeping a close eye (and ear) on when a crowd is present, because their mirrors would have called the police on them ages ago if they had the capability.
I don't know all that much about Nick Kyrgios but I do know that he's not shy about owning the fact that he's kind of a prick. Why would someone that admits to being a prick refuse to admit that he used an obscenity that has audibly come out of the mouth of everyone that has ever picked up a racket? I'll tell you why, because he genuinely doesn't believe that he did. Plus - unless you play for the Kansas City Royals - a "fuck" should be irrelevant hearsay that is without consequence when it's shared between you and your hand.