He's had a fairly easy tournament up to this point. He's only been broken once this tournament and hasn't given up a point during 38(?) service games this tournament.
He seems to be in the form of the Great Federer he once was.
More stats to consider:
Federer leads their head-to-head 17-16.
They met two years ago in the semi-finals at Wimbledon: Federer won in 4 sets.
Federer is 9-0 on grass this year.
Federer has only lost 4 major matches on grass since 2011.
Djokovic is 5-0 on grass this year.
He's also only lost 4 major matches on the surface since 2011.