I actually played a kid recently like this. And for me, it's always tough to stay determined and calm in this type of situation. I played the kid 2-3 times at this point and also have seen him tournaments and he isn't known to be like this.
Actually if anything, I've seen him play a friend of mine, who is a good, aggressive player and win because he always was so calm.
But this time, he was feigning confidence from every part of the court.
Screaming "COME ON!" "LET'S GO!" after LITERALLY every single point. Even on errors (and at this point I was hitting a lot of them, I was playing a poor match) I would hear this, so I'd try to laugh it off on changeovers. Didn't work because it only forced him to scream it louder.
So then I tried to say "Shut up" across the court.
Even worse idea.
Towards the end of the match (at this point, I stopped trying, I was mentally defeated) he even said "COME ON!" to a net cord winner. It was a piss poor effort on my part. I lost the match 6-4, 6-2.
He's a good player, solid counterpuncher, but it was embarrassing from me because I've played lots of matches, lots of tournaments, and to let something like this get to me at this point, it's stupid.