Federer leads their previous matches 15-5. The most recent was in November 2013 at the World Tour Finals. Federer won in 3 sets on an indoor court. Their last 7 matches went to a deciding set. Del Potro won the 1st set in 6 of their last 7 meetings.
Federer handles big servers very well and stays focused so he can take advantage of the few chances he gets. His own serve has been pretty solid this year. He has won titles at the Australian Open and Indian Wells this year. Comfortable win for him against Tiafoe in the last round.
Del Potro took Djokovic to 3 sets twice recently. He beat Haase comfortably in the last round. He faced just 2 break points which he saved.
We have some value on Del potro at the start as I feel Federer is priced too low. I think this one is over in 2 sets though.
Back Federer if he gets broken.
Update: Federer won in straight sets. He was never broken.