On a busy day, Missy Franklin just didn't have enough left in the tank.

At the U.S. Olympic swimming trials on Tuesday in Omaha, the 21-year-old finished seventh in the 100m backstroke, failing to qualify for the 2016 Rio Olympics.

Olivia Smoliga (59.02 seconds) and Kathleen Baker (59.29) qualified, while Franklin finished in a time of 1:00.24.

The race was Franklin's third of the day -- she finished tied for seventh in the 200m freestyle prelims on Tuesday morning, then qualified for the finals of that event just 20 minutes before the 100m backstroke.

"I think I do my best when I'm given a big challenge, and tonight I'm really going to have a big challenge ahead of me," she said early Tuesday. "My warmups have been feeling really great. I think I'm trying a little bit too hard and thinking a little bit too much. It's just about trusting myself and just letting it happen."

Franklin came into the international spotlight in her first Olympics in London in 2012, where she won five medals, including four golds. She is the reigning gold medalist in both the 100m and 200m backstroke, but will not defend her 100m gold in Rio.

Franklin will look to qualify in three more events, including the 200m backstroke on Saturday -- an event in which she owns the world record.

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Missy Franklin failed to clinch her Olympic spot. USATSI