Why do people say he double faulted so much? Fed had only 4 DFs. In 2015, when he steamrolled Nole at Cincy, he had 2 DFs. Are we saying that an extra 2 DFs is so much?
And he more than made up for the DFs with more aces this time. He served 11 aces vs 7 in 2015
The matches tactically are far different in how you also need to interpret the numbers.
More aces doesn't necessarily mean he was serving "better" than 2015. He was going for more on firsts, and taking more chances on 2nds.
2015 v. 2018:
1st Serve Points Won: 83% vs. 67%
2nd Serve Points Won: 73% vs. 47%
Break Points Saved: No BP faced vs. 3/6
2nd Serve Return Points Won: 53% vs. 22%
More aces could mean he was ramping up his firsts more because his lack of mobility to protect his second serve. Not going to be acing a lot on second, but you can go for more kick to end points quicker. If he isn't moving well or is carrying na injury, he certainly doesn't want to be playing extended rallies on the second serves. So it would make sense that he'd double fault more trying to go for a bit more on 2nd serves.
The fact his returning was really bad and had no range or very much stretch on returns would also mean his best plan was to servebot and push to a tiebreak.
I think it's telling when Roger has to serve and volley more and more in a match. In 2017 he found the right balance. Under Edberg he was doing it too much. In 2017 the mix of baseline and serve and volley was probably the balance he's ever used. This week against Stan, Goffin, Mayer, Nole he was serve and volleying a lot more in situations he usually doesn't.