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It wasn't anything close to his best, but defending champion Nathan Adrian survived and advanced to the semifinals of the men's 100-meter freestyle at the 2016 Rio Olympics.

Adrian finished 16th in the preliminary heat with a time of 48.58.

"It was alright," Adrian said afterwards to NBC. "Feeling a little rusty this morning, trying to get back into the meet after a late night. You're swimming for a lane. I'm pretty happy to get an outside lane."

Australia's Kyle Chambers (47.90) narrowly edged U.S. swimmer Caeleb Dressel (47.91) for first place.

Adrian captured gold at the 2012 London Olympics by the smallest margin (0.01) ever and is aiming to become the first U.S. repeat champion since Johnny Weissmuller in 1924/1928.

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Nathan Adrian barely advanced to the semifinals of the 100-meter freestyle. USATSI

Here are the top 16 in 100-meter freestyle. They will swim Tuesday night for a chance at the final, which takes place on Wednesday in Rio.

1. Kyle Chambers (Australia) 47.90
2. Caeleb Dressel (USA) 47.91
3. Duncan Scott (Great Britain) 48.01
4. Cameron Mcevoy (Australia) 48.12
5. Santo Condorelli (Canada) 48.22
6. Joseph Schooling (Singapore) 48.27
7. Damian Wierling (Germany) 48.35
8. Vladimir Morozov (Russia) 48.39
9. Pieter Timmers (Belgium) 48.46
10. Luca Dotto (Italy) 48.47
11. Yuri Kisil (Canada) 48.49
12. Sebastiaan Verschuren (Nederlands) 48.51
13. Marcelo Chierighini (Brazil) 48.53
T14. Ning Zetao (China) 48.57
T14. Clement Mignon (France) 48.57
16. Nathan Adrian (USA) (48.58)