Federer gave the crowd at the Arthur Ashe Stadium a scare during his last 16 match against Kohlschreiber after he left the court when he was 2-0 up in sets.
The Swiss maestro was suffering from a back injury prior to the final Grand Slam tournament of the year, and had to retire from the Cincinnati Masters last month as a result.
In the earlier rounds of the US Open, it was rumoured Roger Federer would withdraw from competition again.
And many had their hearts in their mouthes when the 35-year-old required off-the-court treatment.
But Federer put everyone at ease in hilarious fashion by admitting that he required treatment for his bottom after beating Kohlschreiber 6-4, 6-2, 7-5.
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“I just needed a bit of a rub on my bottom.” Federer said whilst laughing out loud.
“I didn’t want to do it on court.
“I wanted to get it done as quick as possible, so Philipp wouldn’t have to wait.”
Federer had to battle through two gruelling five-set opening matches against Frances Tiafoe and Mikhail Youzhny prior to his straight sets win over Feliciano Lopez.
The win over Lopez extended his unbeaten record against him to 13-0.
And he made it 12-0 with his victory Kohlschreiber at Flushing Meadows last night.
Federer added: “I just felt like I didn’t drop my serve so that was a first in this tournament and to do that throughout the match is a great feeling.
“It’s been a great match for me today against Philipp as he can be very tough, but I protected my serve well and that helped me a lot today.”
Up next for the world No. 3 is a quarter-final clash with Juan Martin del Potro.