Good stuff.
As you can imagine I'm a veteran Thiem watcher so allow me to enlighten on what Thiem was doing in this match on return which was really amazing:
Set 1. Thiem started the match with his typical deep hard court return bunting (returns up close for the bunt) and even on 2nd serve for the most part. He was wildly successful against a coldish Nadal as the time taken away was enough to get Nadal hitting shorter. (Aside: This result was a beautiful mauling which shows what Thiem can do against lesser players or those off. Thiem 100% of first serve points in the first set and allowed Nadal only 7 points. He did similar to Nishikori at RG where a cold Kei won only one return point in the first two sets. Thiem also blanked Anderson for the first two sets pretty much this year at US Open. And then he mauled Delpo last year in that great US Open match for the first two sets. Baseline domination of the highest order.)
Set 2. Nadal came out much, much stronger near the start of the 2nd and started moving much, much better with excellent aggression from the ground. Thiem's return style in the first was not working as well because of this and he started varying return position (going back deep to run around and hit heavy forehands). Overall this return approach was not working and this was despite Nadal still be clearly tight on serve and not hitting it all that hard (average serve speed for set was 101 mph).
Set 3 and beyond. Thiem went with a return style I've never really seen him do much of before. He was trying to crush 2nd returns (something he definitely added to his game in early 2018, but sparingly used). But this quickly morphed into Thiem standing up close on return and trying to hit hard over every ball to take maximum time from Nadal. A lot of these shots were a tad blocky off the backhand of course, but he upped the return pressure immensely and the seeds of this probably started with his late break of Nadal in the 2nd. Nadal responded to this with much, much bigger serving that was clearly on par with his 2017 serving performance. Average serve speed hit 107 mph in set 3 dwindling to 104 mph by the fifth. Nadal served extremely well in sets 3-5 and Thiem had the best returning of his career during this period and more importantly it was different from what he's done in the past. Perhaps you thought Nadal's serve was not great because of the Thiem returning, but it was some of his highest quality in my mind. I'd love to have mph stats from 2017 US Open on Nadal, but I don't see them available any more.
Nadal was a deer in the headlights the first set, but I'll disagree with you and say that matches his best on hard courts in 2017 and as I think about it more. I think Nadal's level in sets 3-5 was higher than anything in 2017 where he did not face great competition at the US Open.
Understanding the greatness of Thiem's adjustments on return I think brings into focus just how well Nadal had to play in this match. Nadal's ground game starting in set 2 had Thiem under tremendous pressure. Thiem was forced to go really, really big from the ground or he would have been wiped out in every neutral rally. It was an amazing turn around by Nadal who was getting wiped out in neutral rallies. Nadal returned Thiem's first serve beautifully from set 2 onwards and had Thiem under immense pressure. Thiem's 2nd serving for this match was surprising heroic given the dire situation in the neutral rallies with Nadal. It was an amazing performance by both.
I agree 100% with you on Nadal tired at the end of the 4th and beyond. Unfortunately he didn't drop much.
His serving still had plenty of sting and high percentage for how hard he was serving. Thiem also was gassed by early in the fifth. You could see it in his biggest strokes where he just didn't have the same control, but luckily settled into some tired, but loose slugging that kept him in the match. One great thing for Thiem is despite being punch drunk he found some first serves especially under pressure. Both played worse in the 5th and were leaking a few errors, but still it was a tremendous battle to the last gasp (that overhead
.)