During that point (it was the 1st point of the 12 game in the 4th set when Andy was serving for the match) Federer thought Andy hesitated a bit thinking the ball was probably long, but then he quickly realized that he had no challenge left, so he hit a passing shot instead. But that little activity confused Federer and he felt he got passed because of it since he thought Andy was going to stop playing and call it. That's when he said, 'f**king stop it'. He was fuming anyway since Andy broke him back and then broke him again and now he was serving for the match. Federer wasn't prepared for a loss clearly. He has no chance against Nadal. Nole is too good at the moment and now Andy is coming into terms, so his chances of winning more Slams are only getting slimmer and slimmer. You can add the Olympics loss as well. And of course Andy was under his skin already by then. Anyway, that's my interpretation of the incident.
I'm not sure I understand any of that, Emma. What I do know is that the expression on Andy's face sums up exactly what he feels about him, better than any words.