The stage saw Impey edge out overall leader Patrick Bevin and reduce the New Zealand rider's advantage to just seven seconds with two days of racing remaining.
The stage came alive on the Corkscrew climb, with Porte, George Bennett, Wout Poels and Michael Woods - the best four climbers in the race - going clear.
It looked as though Bevin was in trouble, but he paced himself and used Impey's Mitchelton-Scott team to bring the race back together on the technical descent.
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