The 23-year-old broke away with 47km remaining, to claim her first UCI Women's WorldTour win of 2017.
"When I attacked I wasn't really thinking. I just chose the moment” Niewadoma said. "It can be a good opportunity to go away.
“I expected somebody to join me, unfortunately, I was alone and I was thinking 'Oh no, there's 50km to go, there's a lot of wind, there are hills. I wasn't sure if I could gain a lot of seconds but I was really motivated to keep pushing.”
Niewadoma built a three-minute lead at one point, but Boels Dolmans worked hard to reduce the deficit, but such was the WM3 Energie rider's spirit that she stayed away to celebrate a solo win.
“Normally races like this end up in a bunch sprint, and I knew that there are a lot of teams here like Ale Cipollini and Sunweb who want a bunch sprint, so I thought they would chase me hard,” she said.
“When I saw the 15km to go sign I knew I had to go full and see what happened.”
Niewadoma leads the general classification by 1min 46sec over Vos, with Majerus a further two seconds in arrears while 15 riders sit at 1min 52sec.
The WM3 rider also leads the points classification, while Wiggle HIGH5 rider Audrey Cordon Ragot claimed maximum points in the mountains classification.