Dancing with the Stars pro Lindsay Arnold may be unable to compete in the semi-finals on Monday.

Arnold injured her knee during rehearsals on Sunday and as of press time she hasn't been cleared to dance with her partner Jordan Fisher. "I was practicing the pro number and I literally just stepped up onto a stair and it felt like my whole leg gave out and stopped working and I thought it was just a one-time thing and then it happened a couple more times, just doing simple things," a tearful Arnold told People.

"It's frustrating because it's the semi-finals and I want to dance it with Jordan, and I just hope this doesn't stop me from doing that," Arnold added.

Since the season premiere, Arnold and Fisher have been fan-favorites to win the Mirrorball Trophy — something Arnold has never done before. (She's the only pro left in the competition who has never won.)

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According to People, pro Sharna Burgess, who was eliminated with her partner Derek Fisher earlier in the season, is learning Arnold's routines in case she's unable to dance this week.

Arnold's injury comes only two weeks after Lindsey Stirling injured her rib during rehearsals. Stirling, another serious contender for the Mirrorball Trophy, performed through the pain and remains one of Arnold and Fisher's toughest competitors.

Dancing with the Stars airs Mondays at 8/7c on ABC.