Nadal used to have one of the best records in five set matches, but since he lost to Fognini at the USO in 2015, it's plummeted. Here are Nadal's record in five sets by percentages:
Before the 2015 US Open: 17-5 (77%)
Since the 2015 US Open : 3-7 (30%)
Five of these losses have been biggies:
USO 2015 Round 3
Wimbledon 2017 4R
AO 2017 F
AO 2018 QF's
Wimbledon 2018 SF
What has caused Rafa to not be as great in five setters in recent years? He's a physical and mental beast, so you'd think that he would hardly ever lose in a fifth set. I tend to think it's age, but are there other reasons his five set record has declined so much?
So this is my analysis of the whole situation:
I agree with you that despite the trend starting since AO 2012 when he lost in 5 to Novak and then proceeded to lose to Rosol at Wimby is where the trend started, but he also snapped that by winning in 5 vs Novak at RG in 2013 so the appropriate mark would be the 2015 USO since when he hasn't recovered his 5 set record.
Like others have mentioned, it's crucial to differentiate between the years in which Nadal has had this record: 2015 and 2016 are known for years where he had a loss of confidence and injuries, 2017 and 2018 are where his resurgence began and has continued. In 2015 and 2016, we have to remember that Nadal was pretty darn mediocre on HCs, in fact from February 2014 to August 2017, Nadal didn't even win a single HC tournament such was his slump on the surface. He was struggling significantly on HCs, therefore there is no surprise to me that he would be vulnerable against the likes of Fognini and Verdasco, players who can hit big with their groundies and are reknown for their shot making as well as their short fuse mentality. IMO the 1-3 record in 5 sets in 2015 and 2016 isn't characteristic of Rafa's career on HCs and it's more a resemblance of his form in those two years where he was playing more like a standard top 10, top 20 player than the ATG we know as Rafael Nadal.
Moving onto 2017 and 2018, a record of 3-4 is still off from what Rafa was accomplishing in 5 setters when in his early to mid twenties but margins are so close that a single point can decide a match and turn a losing record into a winning record (after all the difference between 3-4 and 4-3 is the outcome of one match!). In these past 18 months, Rafa has done a lot better on both grass and HCs than in the preceding 2 years. The 5 setters he lost to the likes of Pouille, Verdasco and Fognini, would most likely be victories either in straights or in 4 sets for Rafa in 2017 form. So on the surface, it seems surprising that Rafa has a losing record again when he's improved but once the matches are analysed there is no surprise as to why he has lost them. He lost the AO Final to Fed and that was hard fought, but eventually the better player prevailed similarly to the outcome of the 2018 Wimby SF. The loss to Cilic and Muller maybe deleterious to Rafa's record but they also display fighting and competitive spirit, in the case of the former, Rafa only relented when the opponent had the upper hand in the fifth set and in the latter, it was commendable for Rafa to battle back from 0-2 sets down and to push it so deep into the 5th set but for his opponent to eventually come through.
With these defeats, often you can simplify and say one factor is more important than others but that's incorrect IMO, a defeat on grass for Rafa is a lot different than a defeat on the slow HCs of the US Open. So, for me his record has declined due to several reasons which I'll go through now: a permanent drop in playing level on HCs, let's say that the Rafa that almost defeated peak Novak at 2012 AO and had his best year on HCs in 2013 is 100% in form, the Rafa from post 2014 AO - 2016 was about 60-70%; he has since recovered his level to about 80-85%, which is good enough to defeat the players that he was losing to between 2015-2016 in slams but not good enough to beat the very best players. A second reason is his physical decline, since the back injury in the 2014 AO Final, I've lost count of the injuries he's suffered throughout the years and without a doubt those have impacted his playing style, as he realised in 2015-16, his old style of grinding players from the baseline wouldn't work as this was the first time in over a decade that he didn't win a slam. This prompted him to adopt a more aggressive style which has helped him against the general field off-clay, but against the best players he's become more vulnerable to opponents who have a better attacking game and that has been exploited by the likes of Federer and Cilic at the recent AOs. A physical decline often leads to a mental decline too and this has manifested itself most notably in Nadal's approach to 5th sets; once upon a time, he could rely on the fact that he knew he would not be the one that makes the error in the rally and that he could not be outlasted but physically he can't do that anymore, he knows that opponents, especially the best have the self belief and the game to overpower him and that he must attack - leading to a spiral of self doubt on his own service games and therefore an increased vulnerability that simply wasn't present in Nadal's physical prime. Lastly, I don't think Muller and a declined Novak would have beat the Nadal between 2006-2010 at Wimbledon, he just had too much game at that time and was confident against everyone once the second week of Wimbledon began - his physical decline, mainly the dodgy knees have led to a string of failures since 2011. So in all, a combination of factors, but mainly a loss of form/level on both HCs and grass, physical decline which has led to mental decline and the reinvention of his game has left him open to getting ramshackled by top players who are better at attacking than he is.