ANNECY, France, July 23, 2009 (AFP) - Alberto Contador said he will remain focused on his job of winning the Tour de France in the coming days despite taking a virtually unassailable lead in the race here Thursday.

Less than a week after his impressive victory on the summit of Verbier in Switzerland which gave him the race's yellow jersey, Spain's 2007 champion claimed his second stage win by winning the 18th stage time trial.

With a 4 minute, 11 seconds lead on second-placed Andy Schleck of Luxembourg, and seven-time champion Lance Armstrong in third at 5:25, it has cemented Contador's reputation as the best stage racer in the world.

But the Spaniard said he will be vigilant on the race's next big rendez-vous on Saturday, when the 20th stage takes the peloton up the 21.1km climb to the summit of the legendary Mont Ventoux.

It is there that attacks could come thick and fast from the likes of Schleck, his older brother Frank, who is sixth overall, and, perhaps, Armstrong, who is aiming to finish in the top three.

"There are still three days of racing on this Tour de France and all of my focus and concentration will be on making sure I finish as the champion," said Contador, who beat Switzerland's Fabian Cancellara by only three seconds to win the 40.5km time trial.

"I can't afford to make any mistakes on the stage to the Ventoux. Even though it doesn't seem like it at times, I've got an extraordinary team with me who will support me."

Although Contador would have to suffer a dramatic collapse to miss out on victory, Andy Schleck has already promised fireworks on the Ventoux's bald slopes as he bids to make it a Schleck two-three on the podium.

While admitting Contador "showed he was the strongest", Schleck warned: "The race is still not over. I'm second overall and my brother (Frank) is sixth.

"We'll be doing what we can on the Mont Ventoux. I think there we can push Frank back up the standings."

With organisers back-loading this year's Tour with potential race-winning stages, their hopes for a suspense-laced finish look set to be achieved - although it appears the fight will be over the podium places.

The Schleck brothers took a positive step towards that aim by combining their climbing skills superbly on Wednesday's final day in the Alps, where Armstrong was left trailing.

Armstrong is currently 1:14 behind Andy Schleck, and he admits he could have his work cut out.

"(To finish) 16th in a time trial is not a good result, but my ambition is to get on the podium so I have to be happy with that," said the American.

"I want to protect my position. With Andy climbing so well, (I) just gotta watch for the moves and don't let him get away."