The Australain road champion launched into the race winning move out of a leading group of 15 riders which formed inside the final 50km of the 130km stage.
The 28-year-old worked well alongside her breakaway companion Elena Cecchini (Canyon-SRAM) and the duo were able to open up a significant advantage of four minutes over the chasers.
In a boost of confidence ahead of the Rio Games, Spratt was too strong for Cecchini in the final kilometre.
"I was climbing really well and feeling really good," said Spratt. "I just saw an opportunity not long after the main climb where it had already began to split up."
"It was a good moment and so I attacked then Elena came across to me and we worked really well together. I think we both had an interest to try and stay away.”
In the final, Spratt shot past Cecchini to take the stage honours and move up into second place overall with just one stage remaining. Cecchini moved from seventh to first on the general classification and has a three minute and five second advantage over stage winner, Spratt.
"It feel's great to win, it was a target for the team coming into this race to go for stage wins and now we have ticked that box.
"For me personally, it is my final preparation before the Olympics and I have had really good feelings in the legs here so I think I am climbing well." - Amanda Spratt
Spratt was also awarded the most active rider's jersey on the podium, another reward for her huge effort.
"I love racing this tour each year, last year I came close to a stage win but didn't quite pull it off so it's nice to turn that around this year and get the victory," she said.
"Today has given me a lot of confidence. It's always good to get a win before a big event."
After consistently performing well together over the previous stage sport director Gene Bates was pleased the riders could wrap up the day with a victory.
"It was a really great day for us after the hard work over the previous days," said Bates. "The move went after the major climb of the day and it was the perfect situation for us."
"It was strange that the two riders opened up such a big time gap with Cecchini being fairly high up in the overall classification, it was real stand off in the chase group with no teams really wanting to commit and take up the chase."
Yesterday’s third place finisher Annemiek Van Vleuten continued her string of podiums by winning the sprint from the chase group to join Spratt on the podium for the victory celebrations.
"We were able to take advantage of that and come away with the stage win which is just fantastic," said Bates. "Annemiek (van Vleuten) also rode on to the podium and Spratt moves up into second place on the overall so we couldn't have asked for much more from today, it was really great day for the team."