Good day everyone!
Given that these top players all have a lot of ability, FITNESS these days, especially on the slower courts from AO Open to Roland Garros, appears to be the major differentiator when both players are in good form and the ability is relatively equal. Right now, Djokovic is above the rest in my opinion.
Here is how I rank the top 10 players fitness based on their most recent play that I've seen. The little v means I think there is a level of difference between that player and the next.
1. Djokovic [1] (He has taken fitness and flexibility to a new level. I don't know what he's doing, but it's working. Ability wise, I place him below Federer and Nadal, and maybe even Murray, but his fitness and flexibility translating into superb movement, stamina, and ability to recover quickly in matches and between matches takes him above that group)
v
2. Ferrer [7] (he is at a lower ability level than most of the top 10, but in his 33rd year, his superb fitness and spirit still puts him right behind the top guys)
3. Federer [2] (Well into his 34th year, he is still physically prepared to outlast everyone except for Djokovic. Right now he is training on clay with young Dominic Thiem and others in the build up to Monte Carlo and the clay season. He has targeted training where he works on something and doesn't let up until he thinks he is just about perfect and has hit his target. Kyrgios said that about Federer when he trained with him last year prior to RG and Wimbledon).
v
4. Nishikori [4] (Under his coach Chang, his fitness has improved probably 3 fold. He is not getting as many injuries, and he has one of the best, it not the best deciding set records in the last two years)
v
5. Murray [3] (his level of fitness and play definitely declined after Wimbledon 2013, mostly due to his back, and then the recovery. His level of play this year finally seems to have mostly recovered, but he is still coming up short against the fittest players)
6. Wawrinka [9] (he fluctuates in fitness, varying from pretty fit (Nishikori level) to looking like he had too many hamburgers, so I put him here)
v
7. Nadal [5] (he might be above Raonic, Berdych, and Cilic, but their games are so different they doesn't need quite the same fitness, for Nadal it is essential). It's a bit sad that I place him here, because I always considered Rafa #1 in fitness for years, probably peaking in 2008, but in 2009 injuries took some toll, and then definitely after his 2010 3 slam year I think he relaxed, and then after his excellent 2013, even more so, to the point where I was almost shocked at his fitness. He had a relatively poor clay season by his standards in 2014, but still managed to win Roland Garros, but that was it, and in 2015, has dropped even further in fitness and match playing levels after mishaps in the latter portion of 2014. I think he needs to get up to at least Nishikori level of fitness and preferably higher to stop these losses to players to whom he should not be losing. He is in danger of dropping from top 10 if there is no improvement in the clay season. I think and hope there will be, but we will see)
8. Raonic [6] (his fitness has improved, but it is not at the same level of the rest, but then is game is much different and he doesn't rely on it as much. Still, I think it wouldn't hurt him to drop a few more.
v
9. Berdych [8] (see Raonic and game style, not really as much improvement fitness wise though, he's a big guy and wouldn't hurt him to be quicker around the court)
v
10. Cilic [10] (like Raonic as well in game style, but still getting hurt a lot, probably needs to try and work on flexibility and some targeted training for problem areas of his body)
That's how I see it.
Please feel free to agree or disagree and come up with your own list and opinions. 
Respectfully,
masterclass