By Julien Pretot
ARRAS France (Reuters) - The Tour de France has not even reached the mountains and Vincenzo Nibali has already built a sizeable advantage over his main rivals, yet hard work still lies ahead if the Italian is going to complete his collection of grand tour victories.
After a mediocre start to the season, Nibali has hit top form on the Tour, winning the second stage in aggressive style before strengthening his advantage with an impressive display on the cobbles in Wednesday's fifth stage.
Chris Froome was the biggest threat to his ambition of becoming the sixth rider to win all three grand tours, but the defending champion pulled out of the race after hitting the tarmac for the third time in two days on Wednesday.
Spain's twice former champion Alberto Contador struggled on the cobbled sectors and now trails the Astana rider by two minutes and 37 seconds.
Belgian Jurgen van den Broeck is the closest GC (general classification) contender in sixth place overall, 1:45 behind Nibali.
"He is obviously riding better than he has the rest of the year," Garmin-Sharp manager Jonathan Vaughters, whose lead rider Andrew Talansky is ninth overall, 2:05 off the pace, said of Nibali on Thursday before the start of the sixth stage.
"From appearances it seems like he is in a much better state than he was earlier in the year.
"I think those two minutes are hard to bring back."
With Froome out of the picture and Nibali in yellow, Astana will have to try to control the race and hold off attacks from Contador, Talansky, Van den Broeck, Dutchman Bauke Mollema and Spain's Alejandro Valverde.
"I think he'll be able to control the race," said Vaughters. "He won the Giro d'Italia last year, that wasn't just pure luck."
Astana team manager Alexandre Vinokourov told reporters: "It's not a problem at all to have the yellow jersey so early in the race.
"For some riders it's too much pressure to handle, but not for Vincenzo.
"It's clear that he is the big favorite of the Tour," the Kazakh added.
Nibali won the Vuelta a Espana in 2010 and the Giro d'Italia last year. If he were to win the Tour, he would join compatriot Felice Gimondi, Contador, Belgian Eddy Merckx and Frenchmen Jacques Anquetil and Bernard Hinault on the list of riders with titles on the three grand tours.
There are still seven hilltop finishes and a 54-km time trial to tackle.
"The Tour is not over at all," said BMC sports director Yvon Ledanois, whose leader Tejay van Garderen is 12th, 2:11 behind Nibali.
"Nibali was the man of the first week but the Tour is won by the best rider of the third week. And I think two minutes on this Tour is nothing."
"It's good enough to stay in the battle," said Vaughters, who believes Nibali's rivals will have to be creative if they are to unsettle the Italian.
"The race is still the same for us. In the end you've got a powerful team, Astana, controlling the race, so I think with Andrew (Talansky) it's the same game - it's looking for opportunities where Nibali is too focused on Contador or bigger names and we can take advantage of that," the American explained.
"Now it's Astana versus everybody else," said Vaughters. "We're going to be looking for opportunities and for moments when Astana are a bit off their game."
(Reporting by Julien Pretot; editing by Toby Davis)