In a select group of six, all that remained after a blistering attack from world champion Annemiek van Vleuten with around 40kms to go on the final ascent of the Côte de Rimiez, Marianne Vos (CCC-Liv) pulled away too early from her fellow breakaway companions with 400m still to the line.
Deignan teared away in hot pursuit and with Vos tiring was able to out lunge the Dutchwoman right on the line.
The Brit attributed her victory not just to her team mates work throughout the race but to her directors in the car, Ina-Yoko Teutenberg and Giorgia Bronzini.
"I think I had an advantage there," Deignan said. "I had two of the best sprinters in the world in the car. (They've) both beaten Marianne Vos in lots of sprints.
"So I had the best advice and their advice was patience so I took it and waited."
"I was just relieved that I won it. I mean what a performance from Trek-Segafredo today every single rider played their part. Me and Elisa just had to wait for the final.
"(Elisa) did the perfect job, forced marianne to sprint early and I took advantage."
Knowing she had to get away on the final climb, van Vleuten sparked several explosions. But the climb was not tough enough and she failed to shake Katarzyna Niewiadoma (Canyon SRAM), Demi Vollering (Parkhotel Valkenburg), Vos, Deignan and her Trek-Segafredo team mate Elisa Longo Borghini.
But the six stayed away to the line, with most of the bunch finally coming home just under two minutes behind.
Niewiadoma, Borghini, Anna Henderson (Team Sunweb), Christine Majerus (Boels-Dolmans), Cecile Uttrup Ludwig (FDJ) and Lotte Kopecky (Lotto Soudal) tried to get away earlier on the 96km journey around Nice, but the race reformed with around 50kms to the line.
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Coming up on SBS: Giro Rosa - Daily Highlights
Stage 1 - Saturday 12 September
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Stage 2 - Sunday 13 September
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Stage 3 - Monday 14 September
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Stage 4 - Tuesday 15 September
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Stage 5 - Wednesday 16 September
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Stage 6 - Thursday 17 September
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Stage 7 - Friday 18 September
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Stage 8 - Saturday 19 September
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Stage 9 - Sunday 20 September
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