As the peloton rolled along the Adige on the way to the Passo dello Stelvio on stage 18 of the Giro d'Italia, there was a certain sense of inevitability about what would unfold on the slopes of the mountain, the highest of this year's race. 

While the battle for the maglia rosa was up in the air and is now even less certain after the finish at the Laghi di Cancano, the relative decline of two-time race winner Trek-Segafredo leader Vincenzo Nibali was only confirmed as the race skirted the Swiss border on Thursday afternoon.

Lo Squalo was among the last to drop from the GC group on the Stelvio, hanging on a kilometre after race leader João Almeida (Deceuninck-QuickStep) was dropped along with several other top 10 contenders, including Rafał Majka (Bora-Hansgrohe) and Pello Bilbao (Bahrain McLaren).

He followed fellow 35-year-old Jakob Fugslang (Astana) out the back 10 kilometres from the summit and wouldn't see the front of the race again, finishing 4:51 down alongside Almeida as 20-somethings Jai Hindley (Sunweb) and Tao Geoghegan Hart (Ineos Grenadiers) fought over the stage win.

While he's known for strength in the final weeks of Giri past, most famously coming back from 4:43 down to beat Steven Kruijswijk and Esteban Chaves in the Alps in 2016 and winning the Stelvio stage in 2017, there were no such heroics this time.

"I have no explanation other than the others are faster; there is little to say," Nibali said to the assembled reporters after finishing the 207km stage. "There is a generational change. It's clear that the others are showing their youth.

"There are few of us old riders left. I think I'm the only one born in 1984 to fight with the top riders, but I'm still here to fight and do something."

While Wednesday's stage 17 in the Brenta Dolomites to Madonna di Campiglio – also part of the first climb en route to the Laghi di Cancano – passed with as much drama as a sprint stage on the Pianura Padana, the cumulative effect of the climbing still had an effect as the riders raced over 2,000 metres of altitude on Stelvio. 

"It was a really hard stage, because even if nothing happened yesterday, the 5,600 metres of difference in altitude today have been felt on everyone's legs," Nibali said.

"We climbed the Stelvio with a strong pace right from the start, with Sunweb pulling. Then, when there were 9 to 10 kilometres to the summit, and we were already at 2,000 metres, there was a further acceleration from Ineos with Geoghegan Hart and Dennis. The small group of the best riders up to that point exploded."

Nibali responded calmly, even though he had no teammates around him to rely upon. As Fuglsang rode alone in pursuit of the leaders, he latched on to the Bahrain McLaren duo of Domen Novak and Bilbao. Later, Bilboa would sweep up the Dane before the trio left Nibali behind five kilometres from the summit.

The Sicilian then spent the next 33 kilometres – down the long descent to Isolaccia and racing along the valley to the second kilometre of Torri di Fraele – alone before the Almeida group made it across to him.

"I decided to climb with my steady pace, one that I could afford in that moment, even though I was alone. I did my best to keep my own regular pace. On the final climb, I found the maglia rosa group and just stayed with the group to the finish."

Standing past the finish among the rifugi near the Cancano Dam after the summit of the climb, Nibali reflected not just on the events of the day, but on the disrupted season. He also refuted the oft-repeated accusation that this Giro has been fought among a weaker group of contenders than usual.

"It was a very difficult Giro for me at the end of an equally difficult year for everyone," he said. "The season, however, was strange: many of us concentrated on particular events, none of us knew how we were going to get here.

"I have read a lot of criticism about this Giro, but I can assure you that the standard in the race were always very high," said the Italian veteran.

"As for the youngsters, there were discoveries like Almeida, who even today did not go slowly. But we are not discovering Hindley today, he had already shown himself to be the strongest, both in numbers with an 1850 VAM on the first part of Piancavallo, and also in attitude."

But while he wasn't shy to praise the new generation that has risen over the past 18 days – Hindley is 24, Geoghegan Hart is 25 and Almeida just 22 – Nibali wouldn't prognosticate on the future of the race.

With the podium trio of Wilco Kelderman (Sunweb), Hindley and Geoghegan Hart separated by 15 seconds, and with three ascents to Sestriere coming on Saturday's stage 20 ahead of the 15.7km time trial in Milan, a lot can still change before the end of the 103rd Corsa Rosa.

"It has been a Giro full of surprises so far," he said. "We still have two more stages for GC gains. Nothing will happen on Friday and on Saturday we can see the race explode again. Who will win the Giro is too hard to say at the moment."