Back on Parisian clay this fortnight for the first time since his semi-final appearance here two years ago, the 2009 champion has shown strong form in his opening two matches, with victories over qualifier Denis Istomin and former world No.3 Marin Cilic.
Federer played down his chances ahead of the tournament having only contested just three matches this year coming into the event.
But his performances suggest he is in better form on the clay than he expected, and he will be a tough nut to crack for Koepfer, the world No.59, who is searching for a maiden fourth-round spot at Roland-Garros.
The eighth-seeded Federer spent over a year out of action as he recovered from two knee surgeries. He is pleased with how his knee has reacted so far and is keen to see how it will feel as the tournament goes along.