By Mary Milliken

RIO DE JANEIRO (Reuters) - If there is anything that could wipe the smile off the face of the obstinately upbeat American swimmer Missy Franklin, it is a 13th place in a semi-final for the four-time Olympic champion.

Swimming's golden girl of the 2012 London Games not only lost her smile, she broke down in tears after a dreadful showing in the women's 200 meters freestyle on Monday that dashed her hopes for a place in the final.

"It's so hard, knowing all the hard work you put in and then to get here and be so far behind," the 21-year-old said, minutes after leaving the Rio pool.

"I'm just so disappointed that I feel like I left my team down," she added.

It was a tremendous reversal four years after Franklin dazzled London, not just with her record-breaking swimming but also with her goofy teenage ways and utterly winning personality.

Franklin set records on her way to four golds and a bronze medal at her first Games. But for Rio 2016, she qualified in just two individual events - the 200 meters freestyle and 200 meters backstroke - at the trials in Omaha.

And she missed out completely on the 100 meters backstroke, one of her Olympic titles.

"I hope I still get a chance to be on that 4x200 freestyle relay, that would mean the world to me, and then I have the 200 backstroke," she said."I am not done yet, I need to keep my head up and I need to keep fighting and that is what I am going to do."

Compounding the pain on Monday was Franklin's belief that she did nothing wrong during the race or in her training.

"That is kind of how my year has gone," she said. "I feel like I have worked as hard as I ever have and it just hasn't been there. It's incredibly frustrating."

While Franklin looks to her remaining races, she seems to be cognizant that she is passing the baton to the new teen queen of swimming, fellow American Katie Ledecky, who qualified for the 200 meters freestyle final in second place.

"I know Katie is going to be amazing in this event," Franklin said.

The 19-year-old Ledecky, no slouch in the happy swimmer category, tried her best to be upbeat about Franklin's prospects.

"I know that she's going to regroup and get ready for the relay and then her 200 back, and I know both those races will be good for her," said Ledecky.

(Editing by Mark Lamport-Stokes)