That was a fairly strong performance from Nadal particularly in the second set to win that 1-6.
The commentators were in awe of his performance and fitness after being out for so long.
Yeah, Nadal looked awesome, he didn't look like he missed a lot of time due to injury. But I have to say that it wasn't Nadal who started playing better in the 2nd set it was Thiem who started playing worse IMHO. I'm not sure why whether due to fitness issues or something else but Domi looked like he was the one who was out for a year with an injury. It was one way traffic in the second set.
I did not watch, I just read the reports about this match. It was probably a successful comeback and as Nadal said, he iss happy to be able to play at this level. When one consider that he has not played a tournament for so long. The preparation was apparently good enough. But it is not just the physical fitness which is important for the first few games. Mental fitness is also important.