The 27-year-old Armitstead has been in superb form this season but has struggled with illness at the women's version of the Giro d'Italia and has pulled out to focus on her 2016 priority, the road race at the Olympics in Rio next month.
Guarnier retained her overall lead on Friday with two stages to go but American compatriot Stevens is closing fast.
The 33-year-old Stevens won her second straight stage, and third this week, with a storming display in stage seven, a 21.9km time trial from Albisola Superiore to Varazze.
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She completed the course in 36 minutes and 21 seconds, four seconds quicker than defending champion Anna van der Breggen of the Netherlands and Italy's Elisa Longo Borghini.
Guarnier, 31, was 29 seconds back but still leads Stevens by 34 seconds in the general classification.
Van der Breggen climbed to third in the general classification but is nearly two minutes back, while US rider Mara Abbott, who was second coming into Friday's stage, drops to fourth.
Emma Pooley was the best-placed British rider, finishing 13th, and is now in 37th place overall.
The 10-day race finishes in Verbania, Piedmont, on Sunday.
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