The 39-year-old had to fight for over three and a half hours and the clock approached 01 before he managed to defeat 59-ranked Koepfer in the fourth set in front of empty stands in Paris, the night before Sunday.
- I do not know if I will play, Federer says after the match about his continued play in the tournament, reports the news agency AFP.
The idea is that the Swiss, who turns 40 in two months, will meet Matteo Berrettini (Italy) in the fourth round on Monday.
But Federer's troubled knee and the nightmare of Koepfer could put an end to it.
- I have to decide if I will continue to play or not.
Is it too risky to keep squeezing your knee?
Is it time to rest?
The 20-time grand slam winner underwent two knee surgeries last year and has played very sparingly for the past 17 months.
He has also made it clear that his main goal this year is Wimbledon in London, which starts on June 28, where he is chasing his ninth title.
- At every match I have to reconsider the situation and the next morning feel for what condition I am in and how my knee is, he says.
The now eighth-ranked Federer had to work hard, but eventually managed to win over Koepfer with the numbers 7-6 (5), 6-7 (3), 7-6 (4), 7-5.