At the end of 31 kilometres of rolling terrain between Saint-Pee-sur-Nivelle and Espelette, we will know who will wear yellow into Paris on Sunday.
Thomas’ time-trialling has come a long way from the days when he used to ride on the back of a tandem with Luke Rowe’s dad in Wales, but he still has work to do to hold off the world champion against the clock, Tom Dumoulin.
Here, we break down their head-to-head record as Thomas looks to defend a lead of two minutes and five seconds.
In the 19 previous times Dumoulin and Thomas have competed in the same time trial, only once as Thomas fared better, finishing seven seconds up on the Dutchman on the 10.1km stage two of Tirreno-Adriatico early last season. Thomas finished eighth on the seafront course, with Dumoulin 13th on a day when the margins were minimal between the top riders.
Both Thomas and Dumoulin were using the race to prepare for the Giro d’Italia, which Dumoulin went on to win while Thomas withdrew following a crash caused by a police motorbike.
What would represent a safe margin for Thomas? Only three out of the 19 previous meetings has Dumoulin taken two minutes or more out of the Welshman, an interesting stat given than in the majority of them Thomas was not riding for his own general classification hopes and, as he put it, occasionally treating the time trials as ‘rest days’ when he could take a breather before resuming domestique duties. The last time the margin was at two minutes, on the 37.5km stage 13 of the 2016 Tour de France, the comparison was hardly fair given Dumoulin was purely hunting stage wins while Thomas raced in support of Chris Froome.
Thomas would give up one minute 50 seconds to Dumoulin in the 54.6km Olympic time trial at the end of that season, but only found out a few days before he would be taking part at all having focused on the road race. With two minutes and five seconds in hand, Thomas will feel confident he needs only avoid incident to keep yellow and become the third British winner of the Tour.
Though a one-to-18 ratio does not look great, Thomas can take confidence from a succession of big results in time trials over the past two years. His first taste of the yellow jersey came with victory on the opening time trial in last year’s Tour, winning the 14km stage around the streets of Dusseldorf by just five seconds from Stefan Kung. A couple of months earlier, Thomas had been second to Dumoulin, 49 seconds back, on the 39.8km individual time trial in the Giro d’Italia, just a couple of days after crashing heavily. He would withdraw from the race with his injuries after stage 13.
And if Thomas was not wearing yellow on Saturday, he would be in the British national champion’s jersey after winning the 39.7km time trial at Kirkley Hall in June.
Team Lotto-Jumbo’s Roglic clinched his second stage victory in this year’s race with his win on Friday. The Slovenian finished 19 seconds ahead of Team Sky’s Thomas who claimed second place to consolidate his grip on the yellow jersey.
Former ski jumper Roglic has now moved to the third position overall in the leaderboard behind the Netherlands’ Tom Dumoulin – who finished sixth on the stage – in second place. Thomas still holds a 1min 59sec lead over Dumoulin and a 2min 24sec lead over Roglic as the prestigious race heads towards the 20th stage.
Team Sky’s Chris Froome is in fourth position overall.
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.