I believe David didn't play that good today, lots of unforced errors and not just on the forehand side but also on the backhand (if I wasn't wrong, I think the errors on the BH were even more!!)
Pretty expected reslt, for me at least, because although David just won a tournament last week and played relatively well against berdych yesterday, I believe he's still not at his best and he feels it and it affects his confidence in his shots. On the other hand Del Potro is at his best right now, his confidence is really high and he's definitely fitter than David.
Best of luck to David in the next tournament, and to Del Potro in the next round (for the sake of the davis cup...and also del potro deserves it, he's a nice player
And to look at the bright side of it...David won a tournament last week and gained some points so he didn't lose as much points after all, and also he's still to play Basel (right?) and Paris