Could've been energy but he looked mentally beaten as soon as he lost his serve. If it's energy, I'm worried for the quality of tennis going forward. The big three have all played 4 and 5 hour marathon 5 setters and the quality of tennis rarely drops.
I'd expect the players in their 20's would be even fitter with advances in sports science.
Edit: And yet again, if the big three play at least two make it to the finals. Is the gap really that big to the next lot or are the current/new generation just a bit ****?