The Colombian star has been the dominant rider in the 2021 edition of the Italian Grand Tour and went into the tough climbing day with a considerable margin over his rivals.
But as the road turned upwards towards the brutal summit finish Bernal, who led the race by almost 2-30 at the start of the day, began to show the first cracks.
Early on the 11km-long, 9.6 per cent gradient climb, Bernal and Ineos Grenadiers looked untouchable as they set a crushing pace on the fluctuating ramps, until Simon Yates (BikeExchange) launched his attack 3km from the summit.
After initially being the only general classification contender strong enough to follow Yates, Bernal suddenly cracked dramatically, and looked barely able to pedal on the steep slopes of the climb.
As Yates continued to ride away to gain time, Bernal lost more time and was joined on the road by his nearest rival Damiano Caruso (Bahrain-Victorious).
The 24-year-old rallied in the final few kilometres, as his team-mate Dani Martínez dropped back to support the race leader.
By the finish, Bernal had lost 53 seconds to Yates but had only dropped three seconds to Caruso, maintaining his race lead with four stages remaining.
Bernal still leads the race by 2-21 to Caruso, as Yates surges two places up the leaderboard into third, now 3-23 off the race lead.
The stage victory went to Dan Martin (Israel Start-Up Nation), who completed the set of Grand Tour stage victories after a tough day in the breakaway.
Martin added the win to his two stage victories in the Tour de France, and his 2020 Vuelta a España stage.
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