Duckworth has one of the most extreme service grip I've ever seen. Much more of a backhand grip than a continental. The grip is great for topspin (and Duckworth has a nice kick serve), but not so good for pace.
This is about to be a first serve.
Wow, when you look at it from that picture it does look very extreme. Perhaps he was trying to slice the first serve but even then it shouldn't be that far. When you watch him play he doesn't hit the ball that hard so taking away potential speed from his serve feels like a disadvantage to me. On the flip side these things are very good on a clay court so that could explain his success but you really have to be able to switch your play when you step on to a hard court or even a grass court for that matter.
I'm sure he will eventually learn to play differently on different surfaces but I can't help but think this is just habit from the clay tournaments he played this year.