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Porte, 37, lost the wheel as the main GC group paced up the day’s first climb with nearly 100km to go in the stage.
GCN commentator Bradley Wiggins said Porte yelled to him, “gastro!”, indicating he was suffering with stomach problems. Wiggins also said he spotted Porte vomiting.
The Tasmanian pushed on for some 20km the back of the race and indicated to Wiggins he would try to see out the stage in what is likely the final grand tour of his career.
Porte eventually called time on his Giro at at 80km to go in what could be a key moment for Ineos Grenadiers and Richard Carapaz.
Porte has been the strongest climber so far for race leader Carapaz throughout this Giro.
Losing the Tasmanian veteran could offer an opening for Bora-Hansgrohe and second-place rider Jai Hindley, who started the stage just three seconds back.
A 12-rider breakaway peeled clear early in the stage, and held an nine-minute lead going into the day’s second climb.