After Derek Hough fell ill, Maksim Chmerkovskiy took the dance floor in his place on "Dancing with the Stars."

Both Derek and his good friend and fellow dancer Mark Ballas came down with the flu, and both required assistance in training their celeb partners.

Maksim stepped in to perform the Argentine tango with Derek's partner Joanna Krupa on Monday night, although Derek did feel up to coming to the show as a spectator. He waited backstage for Maks and Joanna, telling Samantha Harris, "I'm feeling a lot better," and complimenting Joanna, saying, "She did amazing."

On the other hand, Mark was recovered from his sickness enough to be able to dance with his partner Melissa Joan Hart on the live program Monday, but he did ask professional dancer Anna Trebunskaya to help out in rehearsals.

Earlier today, a source close to "Dancing with the Stars" had told ET that Maksim was possibly going to have to dance in Derek's place.