Borghini made the most of her teammates' hard work, marking several moves before her decisive surge with 150 metres to go held off Katarzyna Niewiadoma (Canyon-SRAM Racing) and Grace Brown (FDJ Nouvelle-Aquitaine Futuroscope).
The Trek-Segafredo star's win and bonus seconds mean she now has the same time as Brown at the top of the general classification, though the Australian remains ahead on count back with one stage remaining.
Despite stretching 142.9 kilometres, it was the 5.5-kilometre climb of Black Mountain that proved decisive in the stage.
Lorena Wiebes (Team DSM) first tried to surprise the field on the lower slopes, though Trek-Segafredo made sure to reel her in and keep Ellen van Dijk in control.
Van Dijk's pace, with Borghini on her wheel, soon reduced the peloton to 25 riders with around two kilometres to go, though Brown and Niewiadoma were still in the mix.
Borghini was next to attack, calling BikeExchange-Jayco duo Alexandra Manly and Kristen Faulkner to follow with Brown pacing herself in behind.
Strong headwinds threatened to derail any forthcoming attacks but that didn't stop Joscelin Lowden (Le Col–Wahoo), Ashleigh Moolman (SD Worx), and Niewiadoma trying their luck in the closing stages.
The latter's move was marked by Faulkner, but as the final 200 metres approached, Borghini burst clear of her rivals to take victory on the summit.
"This climb (is) a long one but it's not that steep, so you can't really make the difference," Borghini said.
"Plus, we had a headwind. It was like attacking but hitting the wall.
"I trusted my sprint and went with 150 metres and I felt strong.
"I wanted to pay back all my teammates, who were really committed. This is for my teammates and the staff."
The Women's Tour continues tonight via SBS On Demand, with coverage of the sixth and final stage commencing at 7:55pm (AEST).
