Chris Horner effectively sealed overall victory at the Vuelta a Espana after extending his lead over Vincenzo Nibali in an epic battle on the climb to stage 20's summit finish on the fabled Alto de L'Angliru.
The 41-year-old American resisted a barrage of attacks from his Italian rival, before riding away from him in thick mist just over 1.5km out on a day that will live long in the memory.
He crossed the line 28 seconds ahead of Nibali in second place and will now take a 37-second lead into Sunday's final and largely processional stage into Madrid, where he only needs to avoid a crash to complete his triumph.
France's Kenny Elissonde (FDJ) won the stage after forming part of the day's 32-man breakaway and later forging clear on his own up the savagely steep ramps of the infamous Angliru.
The real action was behind, though, with gradients of over 20 per cent, dense fog and just three seconds separating Horner and Nibali in the general classification all combining to produce a thrilling finale.
Nibali (Astana) launched his first attack 6.3km out and although he opened up a nine-second gap, Horner (RadioShack-Leopard) remained calm and gradually rode back to the 28-year-old's wheel.
Nibali went on to try three more attacks over the next four kilometres, but saw each of them snuffed out one by one by the relentless Horner.
The American then led the duo up the Angliru's steepest section, which reaches 23.5 per cent in gradient, and while he was able to maintain his tempo, Nibali was by now exhausted and fell away a short time later.
Both men had earlier dropped Alejandro Valverde (Movistar) and Joaquim Rodriguez (Katusha) - third and fourth in the general classification respectively - although Valverde later caught back up with the drained Nibali to pip him to third place on the day.
Horner said: "At my age I do not need to wait until tomorrow to let this sink in. I understand how beautiful it is. I love how big a fight Nibali brought to this and how hard and to such a dark place I had to dig to win this race.
"I knew how hard today would be and how much suffering I was in for. Nibali was amazing. To win here among such great champions such as Nibali and Valverde and Rodriguez means so much to me."
Elissonde, meanwhile, continued France's outstanding Vuelta by adding a famous triumph to compatriot Warren Barguil's two stage victories.
He initially moved clear of the original breakaway alongside Paolo Tiralongo (Astana) on the day's penultimate climb, the Alto del Cordal, but when Tiralongo dropped off to help team-mate Nibali on the Angliru, Elissonde was left to lead solo.
He had taken a five-minute lead on to the especial-category final climb and despite the fireworks behind him and brutal gradients, he was able to hold off Horner by 26 seconds.
Stage 20 result
1 Kenny Elissonde (Fra) FDJ, 3:55:36
2 Chris Horner (USA) RadioShack-Leopard, +26secs
3 Alejandro Valverde (Spa) Movistar, +54
4 Vincenzo Nibali (Ita) Astana, +54
5 Andre Cardoso (Por) Caja Rural, +54
6 Dominik Nerz (Ger) BMC Racing, +54
7 Jose Mendes (Por) NetApp-Endura, +1:15
8 Joaquim Rodriguez (Spa) Katusha, +1:45
9 Serge Pauwels (Bel) Omega Pharma-QuickStep, +1:52
10 Thibaut Pinot (Fra) FDJ, +1:59
1 Chris Horner (USA) RadioShack-Leopard, 81:52:01
2 Vincenzo Nibali (Ita) Astana, +37secs
3 Alejandro Valverde (Spa) Movistar, +1:36
4 Joaquim Rodriguez (Spa) Katusha, +3:22
5 Nicolas Roche (Irl) Saxo-Tinkoff, +7:11
6 Domenico Pozzovivo (Ita) Ag2r-La Mondiale, +8:00
7 Thibaut Pinot (Fra) FDJ, +8:41
8 Samuel Sanchez (Spa) Euskaltel-Euskadi, +9:51
9 Leopold Konig (Cze) NetApp-Endura, +10:11
10 Daniel Moreno (Spa) Katusha, +13:11