Just a day before the finale of "Dancing With the Stars" pro dancer Mark Ballas fell to the floor with a shoulder injury while rehearsing with his celebrity partner, Candace Cameron Bure, Sunday in California.

"I'm not sure exactly what happened but I think it kind of came out and went back in," said fellow pro dancer Derek Hough, who was at the "DWTS" studio when Ballas dropped to the floor in pain.

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"What makes me nervous a little bit is that he's had a serious shoulder surgery before, on the other shoulder," Hough added.

Ballas, 27, was rehearsing his freestyle dance with Bure on Sunday when the injury occurred. He was treated at a local hospital and released, according to "Dancing" officials.

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Though Ballas returned to the set with his arm in a sling, it is not known whether he will dance tonight, the first night of a two-part season 18 finale.

Ballas and Bure are in last place, points-wise, going into tonight's final, after Olympic gold medalist ice skating champion Charlie White was voted off last week.

Meryl Davis and Maksim Chmerkovskiy, Amy Purdy and Derek Hough and James Maslow and Peta Murgatroyd are the three other couples still vying for the mirrorball trophy.

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All three pairs say they are also feeling the physical strains of the intense reality dancing competition.

"Everybody's pushing their bodies and boundaries further than they normally would and, in some cases, further than they should," Maslow told ABC News. "I've got to go have a therapist work on my knees and back."

Hough was forced to stretch on the ballroom floor with an ice pack after rehearsing the salsa and freestyle dances with his partner, Amy Purdy, a Paralympian who is dancing on two prosthetic limbs.

"I've got really dangerous legs," Purdy said. "My legs are like, they're weapons. They're metal legs."

Hough, recalled even being knocked out when the pair practiced a lift.

"I literally blacked out for a hot second," Hough said. "I was like, 'Whoa.' I kind of fell down on the floor."

The final "DWTS" performance show airs tonight, Monday, at 8 p.m. ET on ABC. Tuesday's live two-hour results show will air following the one-hour recap special "The Road to the Finals" on Tuesday at 9 p.m. ET.