On the roads of one of his favourite hunting grounds, the Giro di Lombardia, the Bahrain-M4erida team leader attacked during a steep climb near the end to pull away from Roglic, who lost 40 seconds after crashing in the descent towards the finish.
The 232-km stage from Ivrea was won by breakaway rider Dario Cataldo (Astana) ahead of Mattia Cattaneo (Androni Giocattoli-Sidermec) and Simon Yates (Mitchelton-Scott).
“On the Civiglio climb I knew the gap and I knew how much we could lose at the top," Cataldo said. "I was more worried at the foot of the final descent because we had 30 seconds and Mattia wouldn't work with me any longer.
"In the final 1.5km, I managed to catch my breath a bit, save some energy, then I knew I could do a good sprint. I knew my capabilities and those of my rival. I rode from the front. I opened full gas and gave it everything.”
but the real action was just behind as Roglic, (Jumbo-Visma) riding a team mate’s bike following a mechanical problem, could not follow Nibali’s attack in the ascent up to the Civiglio.
Richard Carapaz (Movistar) retained the overall lead and now appears as a strong contender for the title. He matched Nibali's pace and finished with him after being distanced for a brief amount of time in the descent.
Simon Yates (Mitchelton-Scott), who had lost considerable ground in last Sunday’s individual time trial, looked fresh as he attacked three times in the stage to end up with Nibali and Carapaz.
Roglic, however, was the big loser on the day, going straight into a safety rail in the descent from the Civiglio.
The Jumbo-Visma rider’s lead over Nibali and his self-confidence are now dented.
Overall, Carapaz leads Roglic by 47 seconds and Nibali by another minute going into the second rest day.
That’s cycling," Carapaz said about Roglic's day. "One day it happens to a rider, the other day it happens to another one. That’s part of the passion for this sport.
"I had good legs yesterday. I wasn’t at the front by coincidence. In the last week, my rivals still have many kilometres to recover their time. But I hope to be declared the champion of the Giro d’Italia.”
Tuesday’s 16th stage is a potential turning point in the race as it features the gruelling ascent of the Mortirolo pass, even if the climb up the equally demanding Gavia pass has been cancelled because of avalanche risks.