You are asking the million dollar question.
A tough one, but I've got the answer!
I am shocked that Fed hasn't learned a most BASIC lesson yet in tennis.
The "Law of Averages".
It's simple. It goes like this. If you have a great serve like Fed has, and you are serving 65%+ (which he was),
THIS is what you do when you get to 40-15, Match Point....
You serve 1, 2, 3, or 4 'FIRST SERVES' if necessary!!!
You forget about second serves!!!
If your first first serve isn't an ace or puts Djoker in trouble, you've got three left! (first serve)
If your next 'first serve' isn't an ace or puts Djoker in trouble, you've got two left! (second serve)
If your next first serve isn't an ace or puts Djoker in trouble, you've got one left! (first serve)
If your next 'first serve' isn't an ace or puts Djoker in trouble, you're still at deuce !!! (second serve)
In other words you've got four (4) shots to put the match away, easily.
So what does Fed do?
Serving at 2-2, 8-7
40-15. 1st serve, hits nets, falls to Fed's side. OK, no sweat! You've still got three (3) more 'first serve' chances!
2nd serve, he throws in a weak serve to Djoker's F.H. Huge, unforgivable mistake.
Djokers pushes Fed back to the baseline easily and Fed hits ball out on the side.
The weak serve was totally inexcusable. You go for a BIG second serve!
This is match point at Wimbledon for crying out loud!
40-30 Fed gets 1st serve in, but not good placement. Djoker can only push ball to mid-court, Fed steps inside baseline, hits to deuce side, comes to net on a wing and a prayer (not a strong/good approach shot), BUT stays on deuce side of court instead of moving to center! Djoker sees a wide open court and hits to open side for winner!
If you are going to move into net, it is totally inexcusable to stay to one side (leaving a gaping opening) instead of coming to the center.
Total brain fart move by Fed.
40-40 ... the rest is history.