Two big stages stand between him and glory before Sunday’s procession into Paris – Friday’s monster day in the Pyrenees and Saturday’s time trial to Espelette.
Here we look at the hurdles Thomas must clear to become the first Welshman to stand on the top step of the podium on the Champs-Elysees.
A brute of a final mountain stage
The peloton may wish to offer a prayer or two in Lourdes before tackling Friday’s 200km stage to Laruns. The route will take the riders over three-quarters of the so-called ‘Circle of Death’ – the Aspin, Tourmalet and Aubisque, but they are only three of six categorised climbs on a day designed to break anyone already bending.
The Tourmalet holds a special place in Tour history as one of the first climbs ever used – and the one to have featured most often.
When it made its debut in 1910 Octave Lapize, on his way to overall victory, reached the summit first but did so pushing his single-speed bike as he yelled at race organisers: “You are murderers”
Thomas will be well aware that in recent years, several Grand Tours have been shaken up by a testing Stage 19. Just a couple of months ago, Simon Yates’ spell in pink at the Giro d’Italia was spectacularly ended.
He cracked while Chris Froome went on an astonishing 80km solo attack over the Colle delle Finestre to set up a victory which had looked beyond him for most of the preceding three weeks.
In the 2016 Giro, Dutchman Steven Kruiswijk appeared poised to upset the odds and take pink as he went into Stage 19 leading by three minutes, but he crashed into a snowbank on the descent of the Colle dell’Agnello to lose his advantage.
Vincenzo Nibali, who won the stage, took the jersey off Esteban Chaves a day later and claimed overall victory.
And Thomas will have his own memories from the 2015 Tour de France, when he lost 22 minutes on Stage 19 to La Toussuire, tumbling from fourth overall to 15th.
Those were very different circumstances as Thomas was working in support of Froome, but he had been looking to sneak on to the podium until it all went wrong two days from Paris.
Race of truth
Over the course of 31 rolling kilometres, the final General Classification picture will be painted on Saturday. Thomas’ closest challenger Tom Dumoulin is the world time trial champion, and a race against the clock on Stage 20 offers him a shot of snatching yellow.
But Thomas’ own pedigree in time trials should not be overlooked after he won the British title in June. In recent head-to-head clashes between the two, Dumoulin has shown himself capable of taking almost two seconds per kilometre out of Thomas in the discipline, suggesting Thomas needs a cushion of at least a minute to feel secure.
But such estimates are essentially guesstimates at this stage, failing to factor in fatigue at the end of a Grand Tour and the additional motivation that comes with a scrap for yellow – compared to previous time trials where such prizes were not on the line for Thomas.
One thing is for sure – the clock will not lie in Espelette.
Alexander Kristoff didn’t quite have the legs to sprint to a maiden Tour de France stage triumph but he secured a top three finish on Stage 18 on Thursday.
The UAE Team Emirates sprint king and European champion was well placed in the final metres of the finish into Pau, but had to settle for third behind the French duo of winner Arnaud Demare, of Groupama – FDJ, and Cofidis rider Christophe Laporte.
But the Norwegian capped off a strong team performance. After being led in impressively by his teammates, Kristoff, who sits second in the race for the green jersey, battled hard in the closing kilometres to finish third.
It was a notable team effort from the Emirati formation, with the team’s riders controlling the pace of the peloton for a large portion of the race.
He is second in the points standings but remains some way behind leader Peter Sagan.
Commenting on his finish, Kristoff said: “I didn’t really have the legs in the final kilometre. The team did a lot of work, but unfortunately I did not have the fastest legs and that’s it. I hope in Paris I can win.”
The Norwegian now heads into the final three stages, vying to secure a podium place in the sprint classification.
The final day in the mountains is upon the peloton – and what a day it promises to be. There are no less than six categorised climbs on the 200km route from Lourdes to Laruns and the stage could be one of the highlights of what has been an exhilarating Tour de France.
Should a GC contender attack and succeed in gaining seconds on his rivals, it will make the penultimate day of racing – an Individual Time Trial on Saturday afternoon – even more intriguing.
Demare, who had been accused by Germany’s Andre Greipel of holding on to a car to make it through the mountains on Stage 16, silenced his critics with a superb sprint finish to claim the win on Thursday.
“The best answer I could give Andre Greipel was to win today,” the Frenchman was quoted following his victory.
“Obviously what he said hurt me, and was also insulting. It’s regrettable that people doubt my performances.
“I know I’m not the best in the mountains, but I work hard to make sure I get through them.”
Yellow jersey holder Thomas, of Team Sky, finished 31st and remains 1min 59sec ahead of the Netherland’s Tom Dumoulin while his team-mate Chris Froome remains in the third position.
“I think we’re expecting the worst, hoping for the best,” Thomas said.
“I think guys will try to take any opportunity they can. It will be interesting, but we’ve been riding really well all race.”
There was another crash on Thursday with Colombia’s Nairo Quintana and Britain’s Adam Yates among the several riders who were caught.
Slovakia’s Peter Sagan had been the victim of a huge crash himself on Wednesday and the sprint king wants to just get over the finish line after suffering some big injuries.
“I can’t complain, I could be worse,” Sagan stated after the completion of Stage 18.
“After what happened yesterday with the crash, I was in a lot of pain today. I think tomorrow will be much harder, but I consider myself lucky that, after such a crash, I’m still in the race and not sitting at home.
“But it was my own mistake yesterday. There was no motorbike in front of me. I flew into the forest, like a bird.”
The 200.5km long 19th stage of the iconic race is set to get underway on Friday with the final stage set to take in Paris on July 29.