Mr Strel, who has described his swim of the world's second longest river as his "toughest expedition by far" was ordered to abandon his epic journey by his doctor at the end of last week, with a note on his website revealing that the swimmer was suffering from dizziness, vertigo, high blood pressure, diarrhoea, nausea, and delirium by Thursday evening.
But the Slovenian's "obsession" with reaching his final destination drove him to continue on and even undertake night swimming, the website added.
Known as the "fish man", Mr Strel revealed by satellite phone earlier this week that he had also survived encounters with a number of deadly Amazon residents including piranha fish and bull sharks.
The swimmer, who lost about 26 pounds during his journey, was also blighted by sunburn during the early days of his swim, when he suffered from second degree burns to his face.
"He's hit point zero," Mr Strel's son and project co-ordinator Borut told the Associated Press news agency, adding that there would be a ceremony held in Belem today to mark the completion of his father's mammoth swim.
Meanwhile, Guinness World Records has confirmed that it will consider data to determine whether Mr Strel has broken a world distance swimming record for the fourth time.
The distance swimmer has previously completed a 1,866-mile swim of the Danube, a 2,360-mile swim of the Mississippi and a 2,487-mile swim of China's Yangtze river.
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