I thought he served big the entire match, at 69% for the match, and even served 71% in the second set where he got routined. I thought he picked up his ground game which made the difference and allowed him to challenge Djokovic. He also returned better in the final two sets.
There was definitely a difference in his serve. 1st serve percentage doesn't paint the full picture. In the first set, he had 0 aces and 3 doubles. In the second set, he just had 2 aces. 2 aces, 3 doubles in the first 2 sets and 13 aces, 4 doubles in the last two. That right off the bat should tell you there was some difference. There's multiple articles about the match that will tell you he was serving better in the last two sets, whether that's due to better placement or serving bigger, I cannot say as I don't remember the match in that detail.
It was a combo of bigger serving and a much more aggressive ground game that let him back into the match. In the fourth set especially, he was dictating rallies and had Novak on the defensive end. This was a pattern in some of Roddick's matches in the latter half of his career. He only became aggressive when he was down big. Some of the best tennis he ever played at the AO was in third and fourth set vs Cilic in the 2010 QF after he was down two sets.
As for the choke, Roddick served two consecutive doubles when serving for the 4th set up 30-0. He also made a terrible decision at 5 all in the tiebreak when he was dictating the rally, making Novak move side to side and went for a dropshot which he netted. Novak was playing some great defense to his credit but a dropper was a terrible decision in that situation. That said, Roddick making a bad decision or tightening up on big points was a staple in his career so not too surprising.
I probably would favor Novak in the fifth simply because I think he would have handled pressure better.
Except maybe I'm throwing in a bit of hyperbole.
