There can be no doubt about Richie Porte's resilience after the Tasmanian twice paced race leader Chris Froome up Alpe d'Huez on the 18th stage of the Tour de France.

At the bottom of the second ascent of the famous climb on Thursday it appeared Froome was struggling to hold his Sky teammate's wheel.

But the yellow jersey wearer subsequently launched two blistering attacks to dislodge his closest rivals Alberto Contador and Roman Kreuziger (Saxo-Tinkoff).

Froome paid for those efforts towards the top of the climb but by then Porte had clawed his way back to the key group of Froome, Nairo Quintana (Movistar) and Joaquin Rodriguez (Katusha).

The Australian lifted the pace but again it proved too hot for Froome who got on his radio and shook his head before Porte slowed.

"Richie ... put aside all his ambitions in this race to help keep the jersey on my shoulders and he did such a good job today - really fantastic," Froome said after the 168.5km stage won by French escapee Christophe Riblon (AG2R).

"I mean he paced me through that whole climb basically."

With less than five kilometres to race Froome suffered a hunger flat and raised his hand looking for assistance from the team car.

Again it was Porte to the rescue.

"Richie was definitely feeling a lot better than I was today and I was running out of sugars towards the end," the Kenyan-born Briton said.

"I had to ask him to go back and grab me some sugars from the car."

After the stage both Sky riders were penalised 20 seconds for taking an energy gel too close to the finish.

Froome said a mechanical problem with the team car meant they hadn't been able to feed before the 20km mark, as required.

The 28-year-old's early attacks may have contributed to Froome losing over a minute to Quintana and Rodriguez on the stage.

However, the race leader did gain almost a minute on Contador and 1:13 on Kreuziger.

"It was a really good feeling to hear on the radio they had dropped back on that last climb," Froome said.

Saxo-Tinkoff road captain Michael Rogers finished an impressive 13th on the stage to move into the top 10 overall.

The Australia now sits eighth on the general classification (GC) with two more tough Alpine stages to come.

Cadel Evans (BMC) was dropped early the first time up Alpe d'Huez and finished more than 25 minutes behind Riblon in 80th place.

After taking the stage the French winner declared: "I sprinted and the public supported me and I'm in the sky."

Froome now leads the GC by 5:11 over Contador and 5:32 on Quintana.

Porte is 20th at 37:42.