Bradley Wiggins' hopes of victory in the Vuelta a Espana suffered a major setback today as the Briton lost the lead in the General Classification following an inspired ride by home favourite Juan Jose Cobo.
Cobo mounted a decisive late charge on the energy-sapping 144km ride from Aviles to a summit finish in Angliru, the Geox rider finishing in four hours, one minute and 55 seconds after breaking his rivals on the final steep climb up the Alto de l'Angliru.
Cobo trailed Wiggins by 55 seconds at the start of today's 15th stage but turned that into a 20-second advantage at the head of the GC with six stages remaining before the finish in Madrid next Sunday.
Wiggins' fellow Briton and Team Sky team-mate Christopher Froome moved into second place overall after finishing 48 seconds down on the winner.
Wiggins, who struggled as the final climb reduced many riders to walking pace, now sits 46 seconds off the pace in third place in the GC after finishing 1min 21secs down on Cobo, who also earned a 20-second time bonus for taking the stage win.
Ireland's Daniel Martin came 11th, one minute 41 seconds behind Cobo. His cousin Nicolas Roche came 33rd, six minutes 19 seconds off the winner.
Martin is now 16th overall, six minutes 42 seconds off the pace. Roche is 20th, nine minutes 16 seconds behind.
There was little sign of the drama to come as the stage got off to a low-key start, the riders sensibly pacing themselves with the big climbs all saved for the end of the day.
The opening break came with 105km remaining, as Simon Geschke (Skil-Shimano) hit the front, dragging along Andrew Talansky (Garmin-Cervelo) and Dimitri Champion (Ag2R).
The trio, none of whom posted a threat to the GC leaders, quickly established a decent advantage to lead the peloton by five minutes with 60km remaining, but the arrival of the first of the day's demanding climbs was the trigger for the gap to come down.
The Category Two Alto de Tenebredo climb cost the leaders around 90 seconds, and by the time they reached the Category One climb of Alto del Cordal with around 25km remaining, Talansky and Champion had been swallowed up by the peloton, leaving Geschke alone out front.
But Geschke's time at the head of the pack was coming to an end, and when the peloton caught up it was Geox's Carlos Sastre and David Moncoutie of Cofidis trading blows at the front.
But it was the arrival of the final climb of the day, the brutal Alto de l'Angliru that finally shook up the order, with Cobo seeing off the initial challenge of Igor Anton to take command on a climb that peaked with a gradient of 23.5%.
After overhauling Anton with 5.5km remaining, Cobo pulled clear of both his nearest challengers on the stage, and, crucially, Wiggins and Froome, who were back in the pack and visibly struggling.
Yet there was no such trouble for Cobo, who demonstrated stunning strength and stamina over the closing kilometres to claim a victory that makes him the clear favourite for overall victory in the Spanish capital in a week's time.
