From live commentary:
LMAO at people thinking this is Nadal punishing Stan. He had nothing but a few points and some money to play for after the first set, and I doubt that motivates Nadal as much as 'win or go home'. His opponent isn't a weaponless nobody, and Berdych took advantage of a less intense Nadal. Maybe Nadal will play for pride in this last set, and maybe he always intended to let it go to a third if he won the first so he wouldn't beat himself up unnecessarily over three. Who knows? There are many possibilities, and they're all thanks to this damned round robin format. Get rid of it and you also get rid of inexplicable matches like this one.
Also, people who take a result like this as evidence that tanking doesn't happen at this level are very simplistic. It's as if a player is either fully committed to winning a match or they're tanking it, and since true tanks are very rare they'll never allow that a player might have done relatively poorly because they simply didn't bring their A-game for the whole match. This was absolutely a case of a player only showing up for certain periods of the match.
Anyway, was a good match from Nadal in the sets that seemed to matter to him. Berdych always seems to make Nadal look good, so I honestly don't know what to read from his performance going forward. And yes, there was some shitting of pants by Berdych at the end of the match.