Despite having fought off delirium and exhaustion during his 66-day journey, the 52-year-old finished his challenge Saturday -- four days earlier than expected -- The Sunday Times of London reported.
Strel has also swum the length of the United States' Mississippi River and the Danube River in Europe. He started the Amazon swim Feb. 1.
The Amazon fete was designed to raise funds for several causes, including degenerative diseases, and to promote preservation of South America's rainforests and the use of the Internet in the medical field.
Too exhausted and ill to speak with reporters, Strel had his son and expedition leader speak for him.
"I'm extremely happy because we have been working really hard," Borut Strel said. "My dad's lying in his room. He's OK, he's a tough guy."
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