Then how do you explain this year's USO? Obviously Djokovic had more stamina than Nishikori. For someone as fit as Djokovic, you can't use stamina as a main excuse, though it might have affected him to a certain degree. Djokovic simply underperforms at USO and there will always be excuses for his losses.
If it was easy to play AO, RG and Wimb. finals (+ many Masters finals) and then USO final, I guess Borg would have won at least one.
The year Sampras made his only RG SF (1996) he lost in Wimb. QF, winning three consecutive Wimb. before that and four consecutive after, but after usually going out at RG and clay Masters in 2R.
Sampras and Nadal are alike in winning majority of their titles for a half of the season only, Nadal the first half AO-Wimb, Sampras the second Wimb-USO-WTF.
Sampras 2 AO + 5 masters (IW or Miami).
Nadal 2 USO + 5 HC masters (Canada, Cinci, Shanghai), with two of them + USO in 2013. but after 7 months off since wimb 2012 (2R loss) and skipping AO 2013 + Wimb. 2013(1R loss).
Federer and Djokovic are alike in winning the titles throughout the whole year every year.