There’s agony on the dancefloor as ‘Dancing with the Stars’ pro Sasha Farber barely made it to the semi-finals after suffering a debilitating back injury.

The dance pro, who’s partnering actress Justina Machado from TV hit ‘One Day at a Time,’ hurt himself during dress rehearsal on Monday, Page Six is told.

A show source said: “We were not sure if Sasha was going to be able to dance, he managed to hurt his back, and he already has a slipped disc, he was in agony.”

Producers went as far as teeing up show dance pro Brandon Armstrong to take Farber’s place.

The source said: “Brandon was learning the dance right before the show in case he had to step in and dance with Justina.”

Farber managed to join Machado for two dances — and they got a perfect 10 in their contemporary dance.

The duo are competing for a place in next week’s final on the ABC show, alongside Skai Jackson, Nelly, Kaitlyn Briscoe, Nev Schulman and Johnny Weir.