Detti touched the wall at seven minutes, 40.77 seconds with Wojciech Wojdak of Poland claiming silver at 0.96.
Detti touched the wall at seven minutes, 40.77 seconds with Wojciech Wojdak of Poland claiming silver at 0.96 back and Italy's Gregorio Paltrinieri earning bronze at 1.67.
Sun, the gold medallist in the event at the each of the last three world championships, produced an uncharacteristically poor performance to finish 8.10 off Detti's pace.
The 25-year-old Sun, who has already won 200m and 400m golds in Budapest, put his poor display down to tiredness.
"I was too tired towards the end, my arms and legs were very tired," he complained.
"I wanted to test my limits during all the events, but the 200m and 400m took a lot out of me. The competition is very high in those events."
"I spent 70 percent of my energy on the 200m and 400m and I didn't have enough left."
"When I go back home, I will reflect on whether to keep doing the 800m," he admitted.