Chris Froome: These next two days are the biggest of the Tour de France

Chris Froome says that the 2017 Tour de France classification contenders are facing the biggest days so far, Wednesday and Thursday, over the Col du Galibier and the Col d'Izoard summit finishes.

Stage 17 on Wednesday climbs to 2,642 metres over the Galibier and descends 28 kilometres down to Serre Chevalier. The 18th stage, for the first time in Tour history, finishes on top of the Col d'Izoard at 2,360 metres on Thursday.

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Making the days more intense, only 29 seconds separate the top four. Team Sky's Froome leads Fabio Aru (Astana) by 18 seconds, Romain Bardet (Ag2r La Mondiale) by 23 and Rigoberto Urán (Cannondale-Drapac) by 29.

"I believe these next two days are the biggest consecutive days in this Tour de France," the three-time Tour victor told press after a wind-swept day to Romans-sur-Isère on Tuesday.

"It's hard to say how selective they are going to be or if it will be the case the four of us within a half minute chasing each others' shadows, or if it is going to get blown open."

Aru, Bardet and Urán will want to blow it up or risk losing the Tour on the final testing day on Saturday, the 22.5-kilometre time trial in Marseille. That stage suits Froome over the others.

"None of the four of us knows how it'll go," Aru said on the rest day. "We are all more or less on the same level in the time trials, so you will be treated to a big show on these mountain days ahead, stages 17 and 18. There will be many attacks."

In Froome's favour, his Basque team-mate Mikel Landa sits fifth overall at 1-17 minutes.

"Landa and I are both feeling well," Froome said. "My goal this season was to be that way in the third week, and I'm looking forward to these next days now in the Alps.

"I think tomorrow [Wednesday] is going to be a race for us to control. Our guys are feeling good, we had a great recovery, we didn't have to do much riding today expect for the final. We are just looking to the next days, especially in the Alps."

He worries about the 30-year-old Colombian and former Sky rider, Urán. He has finished twice on the podium in the Giro d'Italia – second place in both 2013 and 2014.

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"We are all within 30 seconds, if we all went into the time trial as we are now, Rigoberto Urán would be the most dangerous given he's the best time triallist in that group, but given that, we have to see how he gets through the next stages."

Froome survived a hectic stage 16 where winds blew strongly through the Rhône Valley.

Sunweb exploded the race early for a sprint win by Michael Matthews and Sky, with 16 kilometres remaining, initiated a split that caught Daniel Martin (Quick-Step Floors) and Louis Meintjes (UAE Team Emirates) off guard.

"It was quite a challenging stage, with the selection coming not long after that climb, which meant that the GC guys were far up in the front and ready for that split," Froome added.

"A few guys got caught out, Dan Martin especially because his team-mates were back with Marcel Kittel."