Stevens launched a perfectly-timed attack on the final climb at the end of the 118.6 kilometres route from Andora to Madonna della Guardia to secure the win ahead of Guarnier, who also competes for Boels-Dolmans, and defending champion Anna van der Breggen of The Netherlands.

Claudia Lichtenberg, of the Lotto Soudal Ladies team, finished fourth while two-time winner Mara Abbott, who rides for Wiggle High5, had to settle for fifth.

Abbott, also from the US, had come into the sixth stage of the race as the overall leader but has now been usurped by Guarnier, who leads her team-mate by 46 seconds in the general classification.

"I feel strong and I am exactly where I want to be," Stevens said.

Several members of the peloton attempted to launch attacks throughout the arduous journey, which featured four tricky climbs, with the most significant coming on the penultimate ascent.

Poland's Katarzyna Niewiadoma of the Rabo-Liv team was the rider to take the initiative but she was quickly caught by both Stevens and Abbott.

The duo and the Pole were racing together over the top of the climb as they managed to move into a 2min 30sec lead over the rest of the chasing pack with just 80km remaining.

Gradually, the challengers began to come together to set up a thrilling finale as Guarnier crept her way back into contention, much to the surprise of the rest of the peloton.

The American wasn't able to get past Stevens, who held on for a deserved victory, but Guarnier would have been happy with her day's work as she is now the overall leader.

The seventh stage of the race is comprised of an individual time trial, a 21.9km journey from Albisola Superiore to Varazze.