The former world No 1 has been creating his look appearance in a Grand Slam because the 2020 Australian Open having spent a year out of action with a knee injury.
Federer attempted to play down expectations until the championship, insisting his principal focus was to the grasscourt season.
However he got off to an impressive start with a comfy straight-sets success over Denis Istomin.
This was followed by a difficult fight against Marin Cilic, coming in four places.
Koepfer pushed Federer most of the way together with the first 3 sets moving into a tiebreak but the 20-time Grand Slam winner did enough to progress to the next week of the French Open.
But, Federer shocked everyone when he announced he was withdrawing from the championship to safeguard his body.
The 39-year-old did not show much else regarding the status of his knee at his statement.
But Federer has spoken at length before the Halle Open, indicating his unique album on tour leaves him exempt from describing too much.
"Well, when you retire on the center day between game days, this case prior to Monday, you do not need to hold a press conference and provide long explanations," he explained.
"That is why folks start wondering and pros come from each hole and provide their opinions that I find ordinary.
"Personally I'm completely convinced it was the perfect choice.
"I really don't wish to talk about it because I do not need my competitors know why I did not continue playing.
"I along with my group chose very easily I should not play Berrettini on the following day. Everything went well initially but before the Koepfer game there have been question marks if I had to perform that game or not.