The former Norwegian champion held on to beat Moreno Moser of Liquigas and Alexander Kolobnev of Katusha by just two seconds in the 201.6km race, which is run on a 12.6km circuit.
"That was incredible,” said Nordhaug. “I was riding for Edvald [Boasson Hagen] originally because we thought it was going to come down to a sprint.
"I just tried to sit on the front to cover the counter attacks, but felt so strong in those last 4km that I just rolled off alone."
He split the peloton with an attack inside the final 10km before his final escape late on.
Björn Leukemans (Vacansoleil) was the third rider to catch back up to him before his kick in the last 150m.
"When I saw those three guys were coming I waited a bit and then joined them. I didn’t think I would take the sprint because the other guys in that group were very fast, but I just had unbelievable legs today, so to win was fantastic,” added Nordhaug.
"I only realised I’d won inside the last 30m. I’m really pleased because I’ve taken some big steps this year and had two victories already. To win this race though is the biggest thing I’ve ever done.
"As a team, we’re having an unbelievable season and it’s good to have been a part of that success. Winning the Tour de France gave us all motivation and everybody’s been riding well all year long."
He is considered a decent left-field bet for the world championships, which start with the team time trial on Sunday in Valkenberg, Netherlands.
A further two seconds back were Simon Gerrans (Orica GreenEdge) and Sky rider Edvald Boasson Hagen.
The chasing group led by Björn Leukemans (Vacansoleil-DCM) was only a second behind them.