Italy's Vincenzo Nibali underlined his superiority at the Tour de France by winning the summit finish at Hautacam to all but wrap up the overall victory.
It was a fourth stage victory in the race for the Astana rider, who won in Sheffield, the Vosges and the Alps.
Nibali is seven minutes, 10 seconds clear of Thibaut Pinot, who finished 70 seconds adrift in second to move up to second in the overall standings.
Rafal Majka was third to ensure he will win the King of the Mountains jersey.
It was an impressive display by Nibali, who is on the verge of becoming just the sixth man to win all three Grand Tours - the Giro d'Italia, Tour de France and Vuelta a Espana.
"It's fantastic to have won four stages. I didn't believe I'd do that," he said.
"There aren't many stages till the finish now and my lead is consolidated. I'm pretty serene.
"I've wanted to leave a footprint in the Pyrenees. I wanted to show that I'm the strongest, I wanted the stage win."
|Magnus Backstedt - Former Tour de France stage winner|
|"It is not over for second and third and Saturday's time trial is shaping up to be a monster of a ride. Valverde has a chance, he has improved his time trialling and he is going to be in with a shout if he can pedal the mountains out of his legs on Friday's flat stage."I would bet on Peraud finishing on the podium but unless Pinot has done something with his time trial he's going to find it difficult to hold off Valverde. I think it is done for Bardet but three Frenchmen in the top five is great."|
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The Astana rider was paced over the first of the day's two big mountain climbs, the Col du Tourmalet, by his team-mates before racing clear 8km from the summit finish at Hautacam.
He had one minor scare when he clipped a spectator who was standing in the road attempting to take a 'selfie' photo but he managed to stay on his bike and rode over the finish line with his arms outstretched, knowing he had taken another huge step in his quest to win the race.
None of the riders in the leading group were inclined to follow Nibali when he attacked, knowing that they would be unlikely to either catch him or finish sufficiently far enough in front of him to stop him from winning the race.
Pinot, Jean-Christophe Peraud, Alejandro Valverde and Romain Bardet were more interested in watching each other in their battle to finish on the podium in Paris on Sunday.
Spaniard Valverde started the day with a 34-second advantage over Pinot, who is also leading the young rider classification.
However, with Saturday's 54km individual time trial likely to play a huge part in deciding the podium places, Frenchman Pinot, who is a weaker time trial rider than Valverde, attacked on the ascent of Hautacam and was followed by compatriot Peraud.
|What to expect on stage 19|
|"After three tough days in the mountains, the sprinters will come back to the fore. There might be a few desperate guys, those who are yet to win a stage, who are willing to take a risk on the climb just before the finish to try and break clear, but I expect a bunch sprint in Bergerac.|
|Read Geraint Thomas's stage-by-stage guide|
Pinot knew he would have to try and eke out an advantage and finished 49 seconds ahead of the Movistar rider, but it remains to be seen whether the 15-seconds lead he now holds will be enough.
FDJ's Pinot finished five seconds ahead of Peraud and leads the AG2R rider, who finished fourth on the stage to also leapfrog Valverde, by just 13 seconds.
"I was at my limits," said Valverde. "I'm really, really tired but for the time-trial everything depends on the form of the day.
"If I have good legs, I can take the second place back. It's not a dramatic day, I just had to suffer, hang in there, that's what I did to finish with as much dignity as possible."
There are just three stages remaining in the race. Friday is a relatively flat stage which is likely to finish in a bunch sprint with no overall change in the standings, while Sunday's finale in Paris is likely to offer a similar outcome.
Stage 18 result
1. Vincenzo Nibali (Ita/Astana) 4hrs 04mins 17secs
2. Thibaut Pinot (Fra/FDJ.fr) +1min 10secs
3. Rafal Majka (Pol/Tinkoff - Saxo) +1min 12secs
4. Jean-Christophe Peraud (Fra/AG2R) +1min 15secs
5. Tejay van Garderen (US/BMC Racing) Same time
6. Romain Bardet (Fra/AG2R) +1min 53secs
7. Bauke Mollema (Ned/Belkin) +1min 57secs
8. Leopold Koenig (Cze/NetApp Endura) Same time
9. Haimar Zubeldia (Spa/Trek) +1min 59secs
10. Alejandro Valverde (Spa/Movistar) Same time
General classification after stage 18
1. Vincenzo Nibali (Ita/Astana) 80hrs 45mins 45secs
2. Thibaut Pinot (Fra/FDJ.fr) +7mins 10secs
3. Jean-Christophe Peraud (Fra/AG2R) +7mins 23secs
4. Alejandro Valverde (Spa/Movistar) +7mins 25secs
5. Romain Bardet (Fra/AG2R) +9mins 27secs