Geraint Thomas leads the Tour de France with eight of the 21 stages remaining following the leg between Bourg d'Oisans and Valence.

Friday was an easier day won by world champion Peter Sagan across flatter terrain following three punishing days in the Alps, with the Welsh wizard holding a 1min 38sec lead over Chris Froome and Dutch dynamo Tom Dumoulin third a further 11 seconds in arrears.

So twice Olympic track champion Thomas remains on course to become the first Welshman to win the iconic event when it finishes in Paris next weekend.

But what's now between Thomas and a glass of Champagne on the Champs Elysees next weekend?

What's happened so far?

The 32-year-old took the leader's yellow jersey at La Rosiere on Wednesday but topped that by winning on the legendary climb of Alpe d'Huez the following day.

The race appears to be developing into be a three-way scrap between Thomas, four-time Tour de France champion Chris Froome, and last year's Giro D'Italia king Dumoulin. although the threat of Slovenia's Primoz Roglic and French favourite Romain Bardet cannot be completely discounted.

Geraint Thomas sprints to victory at Alpe d'Huez

Isn't Froome supposed to be leading the race?

Yes. Although Froome and Thomas were supposed to have entered the Tour as co-leaders at Team Sky, the situation appears to have changed.

Thomas, unless it's a clever ploy to reduce the pressure on him, has been telling the media since wearing yellow that he's riding for his team leader Froome.

How has Thomas found himself in the lead?

By keeping out of trouble during the opening week - Froome lost time after going off the road and tumbling into a field - and being the strongest rider so far when the road has gone uphill in the high mountains.

Winning back-to-back stages, pocketing a 10-second bonus on each occasion, means his advantage has steadily grown and, barring mishap, he should still be in yellow entering the Pyrenees next Tuesday for a demanding stage to Bagneres De Luchon.

Why is he doing better this year than in previous Tours?

Because he has been a protected rider in the Sky train, sitting behind a formidable array of workers until the final kilometres.

Thomas did do some work on Alpe d'Huez to set up Froome for an attack but the six-time Grand Tour winner's burst was shut down by Sunweb leader Dumoulin.

Yet his load hasn't been anything like in previous years, when he's been burnt out by his efforts and had orders to ease up after being dropped by the leaders on mountain stages.

Thomas has been riding a clever race and stuck to the back wheel of the dangerous Dumoulin after Froome's attack was snuffed out before out-smarting both of them in the sprint for the line.

Geraint Thomas, wearing the overall leader's yellow jersey, and Chris Froome, second left, ride with teammates during the thirteenth stage of the Tour de France

What does it need for Team Sky to put all their resources behind Thomas?

He needs to remain the strongest rider in the race and extend his lead over Froome to in excess of two minutes.

If Dumoulin stays close to Froome, Sky have a real problem on their plate.

Should Froome go on the offensive and Dumoulin follow, it would be folly for Thomas to sit back and watch the pair disappear into the distance.

For that would be playing into the hands of Dumoulin, who is the world time trial champion and has a 31km race against the clock to fall back on during the penultimate stage.

Sky won't want to be in a situation where Thomas, Froome, or both, have an advantage of only 20-30 seconds over Dumoulin come a week today at Espelette.

Chris Froome, left, and Tom Dumoulin sprint for second during the 11th stage of the Tour de France to La Rosiere

How would Froome react to being told to work for Thomas?

He says all the right things but is a winner and wants to match the five Tour de France victories of Jacques Anquetil, Eddy Merckx, Bernard Hinault and Miguel Indurain.

It was clear on Alpe d'Huez he was still riding to win but the picture should become clearer following Saturday's demanding finish at Mende and Tuesday's mountain fest.

What other challenges lie ahead?

Sunday's stage to the medieval city of Carcassonne shouldn't be too taxing because there's a 51km run to the finish from it's big climb and a rest day on Monday.

Tuesday's stage to Bagneres De Luchon features a technical 18km descent to the finish but the Tour is a war of attrition and riders could pay a price the following day on what could be a fire-cracker.

For race organisers have brought in a Formula One type grid start - the leaders will be at the front of it - for a short and explosive 65km mountain stage to Sant-Lary-Soulan.

That's the last of the mountain-top finishes although stage 19 contains the famous Tourmalet and Aubisque climbs before a high-speed 20km drop to the finish in Lauruns.

Tour chiefs will hope the yellow jersey remains up for grabs the following day at the time trial but Sky will have other ideas.

Whether their best hope is Thomas or Froome should be clearer before then.