For all its great reviews and A-list creative team, the musical “Side Show” became the fastest flop of the fall Broadway season on Friday because of a deadly combination: middling ticket sales and another musical eager to grab its theater.

Those two factors led the musical’s landlord, Jujamcyn Theaters, to threaten to invoke the so-called stop clause in the rental agreement for “Side Show,” according to executives involved with the show. This led its producers on Friday to post a closing notice of Jan. 4, according to the producer, Darren Bagert.

Pushing a show to close is not unusual in the highly competitive world of commercial Broadway, but it’s still jarring, and Mr. Bagert said he had a rescue plan for “Side Show” in the offing.

“We were persuaded to post a notice prematurely, in the middle of a holiday season ticket upswing,” said Mr. Bagert, noting that box office sales had recently improved slightly. “If there weren’t another show clawing at the door, I think we’d still be at the theater,” the St. James.