Chris Froome sensationally moved into pole position for Giro d'Italia glory with a ride for the ages as long-time leader Simon Yates tumbled out of the top 10 on a remarkable 19th stage.
Yates began the penultimate day of serious racing holding a 28-second lead over his nearest rival, defending champion Tom Dumoulin, only to falter spectacularly on the day's first major climb when Team Sky forced the pace and Froome launched the boldest of attacks up the Colle Delle Finestre with 80 kilometres remaining.
Froome - the reigning Tour de France and Vuelta a Espana champion who is seeking to complete a 'Triple Crown' of Grand Tour wins - duly kicked on from a chasing group that featured Dumoulin, winning the stage by three minutes.
Dumoulin came home fifth and now trails Froome by 40 seconds in the general classification, with every other rider at least four minutes back. Mitchelton-Scott rider Yates is totally out of contention after losing more than half an hour on Friday.
Having claimed the Maglia Rosa with arguably the finest ride of his illustrious career, Froome now faces the challenge of holding off Dumoulin on Saturday's penultimate stage, which is followed by a ceremonial finale.
Froome is still awaiting a verdict on the adverse analytical finding he provided en route to winning last year's Vuelta, although both he and Team Sky vehemently deny any wrongdoing.
There was an welcome reminder of the controversy for Froome, who had double the permitted level of an asthma medication in his urine at the Vuelta, as a spectator briefly ran alongside him on Friday carrying a giant inhaler.
IT WAS NOW OR NEVER - FROOME
Reflecting on his magnificent attack and subsequent victory, Froome said: "I just felt good and felt it's now or never. I don't think I've ever attacked with 80 kilometres to go on my own like that. The team did such a fantastic job to set that up for me.
"It was going to take something special today ... to go from fourth to first, I wasn't going to do that on the last climb alone, so I had to try from far out.
"It's great [to have claimed the Maglia Rosa]. Obviously there's still a really hard day [to come] tomorrow but the legs are feeling good and I've been feeling better and better as this race has gone on. Hopefully we can finish this off now."
Prior to the day's main drama, Fabio Aru made an early exit from proceedings, bringing his disappointing race to a premature end.
1. Chris Froome (Team Sky) 5:12:26
2. Richard Carapaz (Movistar) +3:00
3. Thibaut Pinot (FDJ) +3:07
4. Miguel Angel Lopez (Astana) +3:12
5. Tom Dumoulin (Team Sunweb) +3:23
1. Chris Froome (Team Sky) 80:21:59
2. Tom Dumoulin (Team Sunweb) +0:40
3. Thibaut Pinot (FDJ) +4:17
1. Elia Viviani (Quick-Step Floors) 290
2. Sam Bennett (Bora-Hansgrohe) 232
3. Davide Ballerini (Androni Giocattoli-Sidermec) 119
King of the Mountains
1. Chris Froome (Team Sky) 123
2. Simon Yates (Mitchelton-Scott) 91
3. Thibaut Pinot (FDJ) 70
Saturday's penultimate stage essentially decides the event, with the final stage effectively a ceremonial one. It remains to be seen how much Froome has left in the tank.