MEGEVE, France (AFP) - Chris Froome increased his lead at the Tour de France to just under four minutes by winning Thursday's 17km uphill time-trial for the 18th stage.

The reigning champion started slowly but overhauled long-time leader Tom Dumoulin, who won the first time-trial last week, to win by 21sec.

Italian Fabio Aru pipped Richie Porte of Australia by a fraction of a second to take third at 33sec.

Bauke Mollema of the Netherlands and Briton Adam Yates remained second and third overall respectively.

Froome was only fifth fastest at the first time-check 6.5km into the race against the clock but was up to third by the second and had taken the lead by the third.

He timed his effort perfectly to claim his second stage victory of this year's Tour.

He also gained 1min 25sec on Mollema to increase his overall lead to 3:52.

With two more mountain stages to come before Sunday's final stage procession into Paris, Froome is as good as already crowned king of the Tour for the third time.

But the battle for a podium finish hotted up as Mollema in second and Porte in sixth are separated now by just 1:08.

For Nairo Quintana, the Colombian who was second overall to Froome in 2013 and 2015, it was another disappointing day, although he did gain time on both Mollema and Yates.

But fifth placed Romain Bardet of France and Porte both closed in on the Movistar leader, who is fourth.