The Welsh rider’s GC hopes disappeared before the Tour de France reached the high country as he fell behind on an early uncategorised climb before losing further time as the stage went on.
Thomas was one of four INEOS Grenadiers leaders at the start of the Tour de France but has struggled since dislocating his shoulder in a crash on Stage 3, putting in a time trial below his best on Stage 5.
The 35-year-old showed further warning signs as was forced to battle back on Stage 7 after he was dropped, but managed to regain contact with the rest of the overall contenders.
However, Thomas’ troubles really hit home on Stage 8 as the peloton exploded on a tough opening six-kilometre climb with the INEOS Grenadiers rider unable to keep pace.
By the intermediate sprint at the 45-kilometre mark, Thomas was six minutes behind the front of the race as part of a gruppetto that included former teammate Chris Froome as well as green jersey holder Mark Cavendish.
Thomas started the day 13th overall, five and half minutes behind race leader Mathieu van der Poel but fell out of contention for the top of the GC standings and any chance of repeating his 2018 Tour de France success.
"The crash has taken a lot more out of me than I thought," said Thomas. "You always get through some days and talk yourself into it, you’ll get better, but yeah it was a super hard stage, a hard start, I was on the back foot straight away and that was it."
Thomas finished 35 minutes down on the day, rolling over the line next to Primož Roglič (Jumbo-Visma) two Grand Tour winners at the back of the pack after hard crashes early in the race.
"It was a wet day, hard roads, I had no chance to come back," said Thomas. "After a lot of hard work since January, to finish 35 minutes down, it’s not really what I wanted. It’s tough on the head as much as anything else. There have been a lot of sacrifices to get here. I got through it anyway and tomorrow is another day."
The Tour de France continues with Stage 9, a 145 kilometre stage from Cluses to Tignes, finishing atop the summit finish to the ski resort in Tignes. Watch the action on SBS, SBS On Demand and the SKODA Tour Trakcer from 20:30 AEST.