Aru was third, also two seconds behind Bardet, while Froome cracked and ended up 22 seconds adrift of the Frenchman. "I never imagined this would happen".
"The number one priority for us is to not allow anyone to come back into the general classification game if they've already lost some time", the Team Sky rider said.
With his Team Sky team-mates peeling off the front, Froome appeared certain to attack in the closing kilometres.
The final of the six ascents on the menu of the 214.5km stage between Pau and the ski station was too hard for Froome, who had worn the yellow jersey over the previous seven stages. LeMond added that Bardet and Aru should pile the pressure on Froome and not hesitate to play mind games as the three-time champion is still the favorite.
"It's one of the most handsome things that can happen to you in life, wearing the yellow jersey", said Aru, who rides for the Astana team. The Colombian Rigoberto Uran, fourth, is only 35 seconds.

La Mondiale rider Romain Bardet of France wins Stage 12 of the Tour de France Thursday July 13. REUTERS Benoit Tessier
A tough 101km stage from Saint-Girons to Foix could cause further changes to the general classification tomorrow.
British champion Steve Cummings had lit up the stage with a long, courageous solo break but was hauled back in on the penultimate climb, less than 10km from the finish. "My teammates did such an unbelievable job but I didn't have the legs at the end to finish it off".
France's Romain Bardet, third in the overall standings, won the stage.
Cummings had said before the stage started he would look to get in a breakaway as a way to pay tribute to Tom Simpson on the 50th anniversary of his death on Mont Ventoux during the 1967 Tour.
Cummings stayed clear until midway up the Col de Peyresourde - a climb which saw Tour hopefuls Nairo Quintana and Alberto Contador effectively say goodbye to their own chances - but the Briton was caught with eight kilometres of the stage remaining and had to settle for the day's most aggressive rider award. "I committed but I didn't have the legs".
Trump, with Peña Nieto, 'absolutely' wants Mexico to fund border wall
The leaders also discussed regional challenges, including drug trafficking, illegal migration, and the crisis in Venezuela. Trump looked to downplay the appearance of tension in their brief encounter on Friday, calling Pena Nieto a "friend".