The real reason was choking by federer and wrong shot selection at MPs..
Come on man, couldn't you hit just a 1st serve of 200+ kmph ??
Okay you didn't, so could not you hit a BHDTL instead of dropshot when delpo was moving forward to the net??...
And you have one of the best and accurate serves of all time, then why 2 DF in TB??
The real reason is, del potro is only one player who can make federer frustrated as he does reply to his aggressive game by even more aggression so that is the case..
And also, federer was not tired but he was not well prepared like last year's IW (he had very hard training blocks and practice beford IW17).
And that kokkinakis loss was actually a strategical loss as federer did not want to injure himself or burning out, also not to have much pressure and play with free mind in Grass season.