"It sounded just like he was trying to work out some new material, nearly like any time of the last 10 years he would come in at the beginning of a new act", the Cellar's owner Noam Dworman told the Times.
Mo Amer, another comic who was on the bill Sunday, said that for the crowd, "it was like a wow moment".
Louis C.K. became one of the dozens of men who have been disgraced by allegations of sexual misconduct amid the rise of the #MeToo movement.
When asked how he felt about C.K's unexpected appearance, Dworman explained: "This has been very hard for us".
Film critic Scott Weinberg said the fact the comedy club owner made it a surprise appearance didn't give the audience the choice of whether to watch the comedian, but rather it was thrust upon them. We know that it is not right that he shouldn't be able to perform again, but there is no clear time to decide how long is appropriate.
But Dworman also added that several other audience members were delighted by the surprise and sent a standard follow-up email saying they were happy to have caught the set. But he chose to just rip the Band-Aid off'.
Comedian Bris Farley said: 'Louis CK being "banished" from stand-up comedy wasn't some kind of petty punishment, it was a fucking workplace safety issue.
Five women last November accused the comedian of inappropriate behavior. But what I learned later in life, too late, is that when you have power over another person, asking them to look" at your genitals "isn't a question.
In the wake of the scandal, CK was dropped from his development deal by USA cable channel FX, and the release of his pending feature film I Love You, Daddy was cancelled entirely.
Cable television network FX, a unit of 21st Century Fox, also severed its ties with CK and Time Warner's TBS network said it was suspending production on upcoming animated comedy The Cops, which CK co-created and was due to star in.