LottoNL-Jumbo’s Primoz Roglic took victory on stage 19 by 19 seconds, enough to propel the Slovenian into third place overall, 13 seconds ahead of Froome before Saturday’s decisive time trial.
As has been his trademark on this Tour, Thomas put in a late dig in the final few hundred metres, snatching second place and with it six bonus seconds that extend his lead over closest rival Tom Dumoulin to two minutes and five seconds.
Team Sunweb’s Dumoulin may be the world time trial champion, but Thomas will be confident of defending his advantage on Saturday’s 31km race against the clock which will settle the general classification fight.
This was the stage that worried Team Sky more - a 200.5 kilometres test from Lourdes that included over 5,000 metres of climbing and took on the Aspin, Tourmalet and Aubisque.
Thomas never broke as his rivals prodded and probed, but while he looked comfortable Froome was in danger of cracking on the final climb and it was as much as he could do to finish with Thomas and Dumoulin as Roglic cashed in.
The race came to life at the foot of the Tourmalet, as Ilnur Zakarin of Katusha-Alpecin attacked and drew out Movistar’s Mikel Landa, AG2R La Mondiale’s Romain Bardet, and Bora-Hansgrohe’s Rafal Majka.
Sky looked unruffled but over the course of the Tourmalet’s 17km, at an average gradient of 7.3 per cent, the quartet pulled more than two minutes clear, and that advantage only grew as they caught the day’s breakaway on the descent.
Landa was second on the virtual classification at that point, within 80 seconds of yellow, but their advantage would begin to tumble as they started to attack one another on the Aubisque.
As the yellow jersey group closed in, Roglic’s team-mate Steven Kruijswijk and then Dumoulin launched attacks.
Thomas was quick to respond but Froome looked in trouble, at one point dropping a full 30 seconds behind as his legs span furiously and his tongue hung out.
The four-time Tour winner needed the help of the tireless 21-year-old Egan Bernal to pace back on, and as he caught the yellow jersey, Landa and Bardet were being brought back.
A terrifying descent followed as they raced down the mountain in poor visibility. It was in the mist that Roglic attacked, and the former junior world ski jump champion launched himself off the side of the mountain to move into the podium places.
Thomas looked in control as he followed his team-mate down the descent and then pulled ahead in the final few hundred metres, hoovering up six bonus seconds to pad what looks to be a commanding lead.