By Julien Pretot

MEGEVE, France (Reuters) - Britain's Chris Froome edged nearer retaining his Tour de France title when he won the 18th stage, a 17-km mountain time trial, to extend his overall lead to almost four minutes on Thursday.

The Team Sky rider, aiming to become the first to retain the title since Miguel Indurain in 1995, clocked a best time of 30 minutes 43 seconds on a course featuring the punishing Cote de Domancy, a 2.5-km climb at an average gradient of 9.4 percent.

He beat time trial specialist Tom Dumoulin by 21 seconds and Italian Fabio Aru by 33 seconds.

"I really didn't expect to beat Tom today, pacing was key. I started off steady and really controlled that first part then gave it everything I had," said Froome.

"I've got a great advantage now."

Five-times Tour champion Eddy Merckx said the Briton already had the Tour sewn up.

"I can't see any reason why he could not keep the yellow until Paris," the Belgian said. "His rivals are just sitting in his team mates' wheels. Nobody can beat him at this Tour."

Dutchman Bauke Mollema, who started the day 2:27 behind Froome in the general classification, lost 1:25 and trails the defending champion by a massive 3:52 going into two final stages in the Alps before Sunday's parade to the Champs Elysees.

Froome's compatriot Adam Yates is third, 4:16 off the pace, as the race to the podium is set to heat up with at least five riders still in the mix for second place.

Colombian Nairo Quintana, runner-up to Froome in 2013 and 2015, had another tough day in the saddle, losing 1:10. The Movistar rider is fourth overall, 4:37 behind Froome.

On his heels is Frenchman Romain Bardet, 4:57 behind after finishing a surprise fifth on Thursday, while Australian Richie Porte, fourth on Thursday, is sixth three seconds further back.

"I'm happy it's a good time trial, I've got good sensations, it was pleasing. There are still two big stages left. I hope I'll find a good terrain to attack," said Bardet.

CAUTIOUS START

Froome started cautiously but finished strong to claim his second stage win in this Tour, raising his fist in celebration after crossing the line.

"As always in the time trial the pacing strategy is critical, particularly in one like today where it was very easy, with that first steep ramp, to go out a little bit too hard and pay for it at the end," said Team Sky manager Dave Brailsford.

"So you have to be brave on a course like that and hold a little bit back to start with so that you can get all your effort out over the course."

Froome was one of few riders to use both a time trial bike and a rear disc wheel, which played a part.

"We went for a very lightweight TT bike, disc wheels, I think Chris was one of the only riders to do a disc but we spent a lot of time doing the maths and the calculations and it looked like it came out right thankfully," said Brailsford.

Friday's 19th stage is a demanding 146-km mountain trek with an uphill finish at Saint-Gervais Mont Blanc.

(Reporting by Julien Pretot; Editing by Ken Ferris)