The 2014 champion won this year's Giro d'Italia and the Tour is, in theory, a step towards the Rio Olympics where Nibali will be one of the favourites for road gold.
History suggests priorities can change, however, and pledges to ride for a team leader can be forgotten if glory looms.
In 1986 Bernard Hinault said he would work for La Vie Claire team mate Greg LeMond but broke his promise. LeMond still managed to become the first American to win the Tour.
More recently, Alberto Contador and Lance Armstrong in 2009, and Bradley Wiggins and Chris Froome in 2012, had rocky relations on the Tour.
Paolo Slongo, Nibali's coach, would not rule out any scenario for his rider.
"His goals this season were the Giro d'Italia and the Olympics but a rider like him, on the Tour, is a bit free in the opening stages," Slongo told Reuters before the start of Tuesday's fourth stage in Saumur.
"He starts the Tour as an aide to Fabio Aru since his programme is revolving around the Olympics. He's ready to help Aru, it's normal."
The Olympic road race will be held on Aug. 6, 13 days after the end of the Tour and riders are expected to peak during the Tour and hold their form for Rio de Janeiro.
"He won the Giro so he starts here without any pressure for a result," Slongo said. "He has no pressure from Astana. He is extremely serene, taking it day by day, without any stress."
Freed from pressure, Nibali, who was back to his flamboyant best during the Giro, could be dangerous.
"Let's see how it goes in the Pyrenees, see where the big guns are," he said.
If Nibali, who is nine seconds behind the main favourites, including Froome and Aru, after four stages, does not lose too much time between Friday and Sunday, anything could be possible as the Italian is set to peak towards the end of the race.
"He's not at 100 percent yet because of the Giro d'Italia. He should be at his best towards the end of the Tour," said Slongo. "We don't know how far he can go."
The 26-year-old Aru, who won the 2015 Vuelta, might find out soon.
(Reporting by Julien Pretot; Editing by Martyn Herman)
