Vincenzo Nibali is making a patchy start to the defense of his Tour de France title, and if Saturday's eighth stage is anything to go by it may not get much better in the mountains.

Nibali lost another 10 seconds to race leader Chris Froome, the 2013 champion, on the short and sharp final climb up Mur de Bretagne, setting off alarm bells in the Italian's Astana team. While the stage was won by Alexis Vuillermoz to give France its first victory in this year's race, it was the way Nibali was dropped on a distance of just 2 kilometers (1.2 miles) that had people talking.

"I don't understand what happened. Hopefully this was just a bad day, otherwise this Tour will be difficult," Astana manager Giuseppe Martinelli said. "Our team should deliver a perfect performance in tomorrow's team time trial, we can't make more mistakes."

Nibali already trails Froome by 1 minute, 48 seconds and riders have yet to tackle the high mountains.

Two-time champion Alberto Contador, former runner-up Nairo Quintana, and American rider Tejay Van Garderen also helped themselves to 10 seconds on Nibali.

In the overall classification, this means that Froome still leads Van Garderen by 13 seconds, Contador by 36 and Quintana by 1:56.

"I guess we're going to have to see how Vincenzo goes up on the long climbs, it hasn't been a great start for him but things can soon change," Froome said.

Before those climbs, starting Tuesday with the first of three consecutive days in the Pyrenees, the weary riders must tackle the team time trial — where Froome's yellow jersey will be under serious threat from Van Garderen, who is shaping up as a dangerous outsider.

Times in a team time trial are taken from the fifth rider crossing the line. The undulating 28-kilometer (17.4-mile) route finishes with a 1.7 kilometer (1.1 mile) ascent.

BMC has four riders from the team that won the event at last year's world championships: Van Garderen, Rohan Dennis, Daniel Oss and Manuel Quinziato.

"With the team I have and what they have shown so far, we are pretty confident," Van Garderen said.

The Australian rider Dennis is particularly quick, having won the Tour's opening individual time trial in a record average speed. At the Criterium du Dauphine last month, BMC won the team time trial with Froome's Team Sky placing sixth.

"I really see as much as 20 or 30 seconds won or lost (between contenders)," Froome said. "It's going to go down to who can deliver five guys fresh enough to get off that final climb."

Saturday's flat 181.5-kilometer (112.5-mile) ride, starting in Rennes, rolled through the Brittany countryside past the imposing Chateau de Hac on the Cotes-d'Armor, a 15th-century castle on the coastline.

As they started to climb, thousands of raucous fans — many of whom were waving the black and white flag of Brittany — cheered them on loudly.

Vuillermoz made his move, and Froome counter-attacked about 850 meters from the end and briefly moved into the lead. But Vuillermoz responded well to give French riders a first win on this year's Tour.

"It's always been a childhood dream of mine to win a stage on the Tour," said the 27-year-old Vuillermoz, a former mountain bike specialist who finished third on Monday's 3rd stage. "I lost my father three years ago. I was thinking of him a lot today and I hope he's proud of me."

Dan Martin was second and Alejandro Valverde placed third.

Vuillermoz enjoyed his brief bout with Froome, who rolled in 10 seconds behind him in eighth place.

"In my mountain bike days, when I was next to a great champion I wasn't scared to attack them," Vuillermoz said. "I've kept that audacity in road racing."

Meanwhile, Nibali crawled up at a sluggish pace to finish 30th.

"I was not feeling so good," Nibali said. "I could not respond to the acceleration."

Monday is a rest day, and after an intense start to the race marked by three big crashes, strong side-winds and searing heat, it will be much needed.

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AP Sports Writer Samuel Petrequin contributed to this report.