He finished more than a minute ahead of Belgian wonder kid Remco Evenepoel,while Italian Filippo Ganna was a distant third,almost two minutes behind the Australian.
The South Australian,who was riding a bike sporting no branding or Bahrain-Merida team logos,was hugged by his teary wife Melissa Hoskins,a former cyclist who wore a"Team Dennis"hat and carried their baby son after he crossed the line.
"It's been a long route to get here since July,"Dennis said.
"There are a lot of people to thank. It's not just been tough for me,it's been tough for them too."
He said he didn't have a lot of confidence heading into the race until he nailed a training session earlier this month September.
"I didn't go without a lot of preparation:a lot of time at home,a lot of work on my head to get myself mentally prepared for today,"Dennis said.
"I knew what pace I went out with last year,so I just stuck on that and got into my rhythm.