It is because he didn't want to give up mentally, he wanted to be beaten physically. When you are mentally switched on, you do whatever it takes to stay that way, and Novak needs this mindset of going out to win every match no matter what if he wants to win RG again. Losing physically here was easy to accept and rationalize - the schedule, the double duty, the night to day sessions, the six hour matches, the less time to recover etc. It is almost a way to give yourself mental therapy that that is what it is taking to beat you out there, and that isn't going to be the case every single time.