Movistar’s Costa, who won in similar circumstances in Gap on Monday, overtook a spent Pierre Rolland (Europcar) with 20km to go on the final climb of the day before holding off four chasers led by Andreas Kloden (RadioShack-Leopard) during a wet and challenging downhill finish.
The 26-year-old double Tour de Suisse champion soloed home 58 seconds to the better of German veteran Kloden to secure the third Tour win of his career.
Race leader Chris Froome (Team Sky) completed the 204.5km stage from Bourg-d’Oisans in a select group alongside his main rivals to preserve his yellow jersey with two days of the 100th edition of the race remaining.
Froome leads Spain’s Alberto Contador (Saxo-Tinkoff) by five minutes and 11 seconds in the general classification, with Costa’s Colombian team-mate Nairo Quintana a further 21 seconds down in third place.
“It was my target to be in the break today but I didn’t think I’d be in a position to contest the victory at the end,” said Costa after his sixth win of the season. “I didn’t feel as strong as I did in Gap so I waited until the final climb - and this time there was still someone out ahead of me.”
That man was Rolland, who started the last of five climbs with a minute over a group of 19 riders in pursuit despite suffering from serious cramps. The Frenchman faded fast as soon as the heavens opened inside the final 25km but moved within one point of Froome in the king of the mountains classification.
The flat opening few kilometres of yet another brutal stage in the Alps saw around 40 riders break clear off the front of the peloton, with Canada’s Ryder Hesjedal (Garmin-Sharp) and Jon Izaguirre (Euskaltel) riding up the road to take an early lead.
A small chasing group formed on the long HC climb of the Col du Glandon featuring stage 18 winner Christophe Riblon (Ag2R-La Mondiale) and fellow Frenchman Rolland.
Hesjedal and Izaguirre crossed the summit of the Col du Glandon with two minutes over the Riblon chasing quartet and another 45 seconds over the large break. The peloton, led by Froome’s Sky team-mates, crested the summit around eight minutes in arrears.
After a long descent, the second of two HC climbs - the Col du Madeleine - saw a further shake up with Izaguirre being dropped by Hesjedal and Rolland riding clear of the small chasing group. Rolland caught and passed Izaguirre with ease before joining Hesjedal 4km from the summit of the leg-sapping 19km climb.
The leading duo combined well with Hesjedal allowing Rolland to take maximum points over the summit before taking the front seat on the fast downhill, which featured 26 hairpin bends and some stunning sweeping vistas. Hitting the bottom, their advantage was up to four minutes over a chasing group of 20 riders, and a huge 12 minutes over a rather lackadaisical peloton.
A long flat drag to the foot of the next climb proved too much for the 2012 Giro d’Italia winner Hesjedal, who was dropped by Rolland at the start of the Cat.2 Col du Tamie. With stage wins to his name from the previous two Tours, Rolland was eager to open his - and his team’s - account in the 2013 race, and in the process put himself back into the reckoning in the battle for the polka dot jersey.
Riding on his own, but in the company of the thousands of cheering French fans lining the road, Rolland crossed the summit with a 1:50 gap over the chasing group with 60km and two more climbs remaining before the finish.
Despite suffering from cramps that saw his advantage come down to just one minute ahead of the next climb, Rolland found a second wind on the Col de l’Epine to cross the summit 1:54 ahead of the ever-slimming chase group - which contained some big name riders such as Damiano Cunego (Lampre-Merida), Dutch national champion Johnny Hoogerland (Vacansoleil-DCM), Spaniard Daniel Navarro (Cofidis) and two former stage winners from earlier in the race, Jan Bakelants (RadioShack) and the eventual victor, Costa.
It was Navarro, a former team-mate of Contador, who made the first attack from the chasing group on the final climb of the day, the Cat.1 Col de la Croix Fry, just as the rain that had threatened the whole stage finally came down.
The Spaniard’s attack thinned out the chasing group, and provided Costa with the springboard to make his decisive attack with 20km remaining. The Portuguese all-rounder caught Rolland one kilometre later, and despite the Frenchman’s best efforts, he was not able to hold onto the fresher Costa’s wheel.
Team-mates Kloden and Bakelants joined forces with Navarro, Spaniard Mikel Nieve (Euskaltel) and Belgian Bart de Clercq (Lotto-Belisol) in pursuit of the lone leader. But as the deluge continued, they could make up no ground on Costa, who crossed the final summit with 13km remaining with a minute to play with.
Avoiding disaster on the wet roads, Costa rode an accomplished descent and kept his concentration until the final kilometre when he started to celebrate with his team directeur sportif.
Kloden took second place almost a minute ahead of team-mate Bakelants, who crossed the line ahead of Frenchman Alexandre Geniez (FDJ) and Navarro, who moved into the top ten on GC.
Back with the main peloton, Froome was tested just the once on the final when Joaquim Rodriguez (Katusha) put in an attack alongside Quintana and Contador - but the 28-year-old race leader kept his calm to cross the summit alongside his rivals. The main pack crossed the line nine minutes down on Costa.
“Today was one of the hardest stages of the Tour with more than four and a half thousand metres of climbing. But every day is hard for us and we just have to follow all the attacks,” said Froome.
“The team worked really hard today and we now have 125 kilometres to go until we can start to relax.”
Saturday’s short and sharp penultimate stage features six categorised climbs including the HC summit finish at Annecy-Semnoz, which is being raced for the first time in Tour history.
If Froome can get through the 125km stage then standing atop the podium in Paris with the yellow jersey will be a certainty.
But while Froome's overall victory seems a formality, the battle for the remaining two podium places should provide the fireworks, with Contador, Quintana, Roman Kreuziger and Rodriguez all separated by just 47 seconds.
- Sports & Recreation
- Chris Froome
- Pierre Rolland
- Alberto Contador
- Andreas Kloden
- Ryder Hesjedal