Djokovic is mentally weak (relatively, compared to some other greats). There was a time were he looked like having balls of steels but in fact he was only riding his momentum and relying on a few lucky shots here and there. 2011 has clearly shown to be an anomaly in the career of Djokovic, all the other years showed the real Djokovic. A very talented player, but underperforming due to his mental fragility. The worst of his losses are the ones against Murray, who is several tiers below him. Federer never lost to Murray in a slam final, and I bet Nadal wouldn't either if he met Murray.
I believe few posters like NatF actually got it right, simply put Nole isn't as great a player among the ranks of Fedal. He has achieved what he is capable of. We are attaching too much expectation to Nole. We do not ask questions like why didnt Stan win WTF etc. Mind you playing along Roger, Rafa and Andy doesnt help either. We really have to think Nole is not the only really good player out there.
Roger and Rafa many not lose to Andy in a Slam final but they also may not lose to Andy in any stage of a GS, because they are that much better in terms of skill. Djokovic isn't that much. That said I do not think it's the big match temperament that's costing Djokovic here. I have no reason to believe he is mentally fragile. He is one of the strongest I have seen in that department. I really do not know what you exactly mean by his mental fragility. Few points:
1. Nole does choke. Everybody do. Who doesn't. We talk highly of how great a fighter David Ferrer is. He chokes too. Rafa choked in RG SF '13, Nole in AO F '12 & WC F '14 all choked a bit in closing out the match in the 4th set. In fact I can mention a lot of great matches where players choked.
2. Nole doesnt start matches like he got owned. He is one of those guys who believed Rafa was beatable on clay and Roger was going down at such a young age. Roger always played like Rafa was in his head. Even Rafa had a problem in bringing his best against Nole for sometime.
3. Few match statistics how Fedalovic fare in big moments. These are percentages, not absolute figures.
- TB record: Fed > Djoker > Nadal
- Final matches: Nadal > Djoker > Fed
- After winning 1st set: Djoker > Nadal > Fed
- After losing 1st set: Nadal > Fed > Djoker
- Deciding set: Djoker > Nadal> Fed
- 5th set: Nadal > Djoker > Fed
- Break points saved: Fed > Nadal > Djoker
- Break points converted: Nadal > Djoker > Fed
- Match won while saving match point/Match lost while having match point: Djoker > Nadal > Fed
I believe these little stats show who has shown better mental strength, simply put: Nadal > Djoker > Fed. What separates Nadal and Djokovic in the above stats is that in some of the categories Federer is too below in all time list, while Nadal and Djokovic doesn't fare as bad anywhere.