Federer R. vs Del Potro J. prediction

Although Federer is in great form this year I expect tough game against a rival who knows how to win against him. Federer played two five setters against weak players in four matches so far at this competition. Head-to-head on this kind of surface in seven out of last eight encounters both players have won at least one set. On the other side Del Potro showed great tennis against world number 8 Thiem where he won in five sets. At the moment Del Potro is underestimated of the bookies and I think this should be over 3.5 sets.