I don't think it was tactics. He had a somewhat off day with too many unforced errors. I think he wasn't feeling the ball, and wanted to finish points early because the longer the point went, the worse he fell. On one side because his shots weren't hurting Djokovic, on the other side, because Novak played a fantastic match, and the more shots a rally had, the more unconfortable he'd be.
For those mentioning conditions... if he played like he did at the end of the match vs Schwarzman, even with this conditions he would have beaten Djokovic, probably. And I say probably, because, even if Nadal made too many mistakes, he did not lose the match. Djokovic won it. He played superb, went for the match and earned it.
The fact that Djokovic was playing so good and Nadal had so many unforced error, yet he was so close to getting that 3rd set, speaks volumes about how good Nadal is at RG.
What makes it not that hard to digest is, that Djokovic played much better and deserved to win, and that for most of the match he kept his composure (same with Wimbledon 2019). What I hate about him is how he sometimes behaves oncourt, which makes me dislike him very much. But, he behaved very good in the match (even though he yelled a couple of times at his box) and brought such high level tennis to the match. Props to him. I hope he doesn't win the tournament, but as a Nadal fan, I'm at peace with the result.