Tao Geoghegan Hart claimed a fifth stage win for Ineos-Grenadiers in this year's Giro d'Italia when he won the 15th stage on Sunday, a 185-km mountain trek, as Joao Almeida's overall lead was whittled down to just 15 seconds.
Britain's Geoghegan Hart was the freshest man at the end of the final ascent, a punishing 14.5-km effort at an average gradient of 7.8%, to beat Dutchman Wilco Kelderman by two seconds.
Kelderman's Sunweb team mate Jai Hindley of Australia was third, four seconds off the pace, after pacing the leading trio up the climb.
Portuguese Almeida came home fourth, 37 seconds behind the winner. He retained the overall race leader's Maglia Rosa jersey, with a 15-second lead over Kelderman.
It was a bad day at the office for double champion Vincenzo Nibali, however, who dropped to seventh overall, 3:29 behind Almeida after cracking with 8.8 kilometres left on the last ascent.
Nibali has lost two key team mates in two days after Giulio Ciccone abandoned with bronchitis ahead of Saturday's time trial and Gianluca Brambilla pulled out during Sunday's stage with knee pain.
Ineos-Grenadiers lost team leader Geraint Thomas to a crash early in the race, but they can now count on Geoghegan Hart to mount a challenge in the final week.
The 25-year-old jumped to fourth overall, a distant 2:57 off the pace. But his apparent superiority on Sunday made him a force to be reckoned with, with three more summit finishes left in the race.
Ineos Grenadiers had already won stages through Filippo Ganna, who has claimed the race's two time trials and a mountain stage, and Jhonatan Narvaez.
"Crossing the line first is something truly incredible," said Geoghegan Hart, who dedicated his victory to Frenchman Nicolas Portal, the team's former sports director who died suddenly earlier this year.
"This one was for Nico Portal."
Monday is a rest day on the Giro.