Thomas attacked when Ineos team-mate Egan Bernal was already free up the road, raising some eyebrows from followers. The Thomas group regathered descending down the Galibier, however Bernal finished behind stage winner Nairo Quintana (Movistar) with an important 32 seconds on the general classification men.
He now sits 1-30 minutes behind Julian Alaphilippe (Deceuninck-Quick-Step) and takes Thomas's second place spot. He is just five seconds ahead of Thomas in third overall.
"Unfortunately we ran out of guys and it didn't seem quite hard enough so the call was made for Egan to jump," Thomas explained. "Hopefully that would kick it off – but it didn't really.
"We were on a nice road, they were kind of just riding tempo again in the group I was win. That's when I had a little dig to just see what would happen.
"They obviously followed me over the top, but I think it was a good day for Egan gaining some time."
Thibaut Pinot (Groupama-FDJ) and the others with Steven Kruijswijk (Jumbo-Visma) and Emanuel Buchmann (Bora-Hansgrohe) followed. Alaphilippe rejoined on the descent to Valloire to save his yellow jersey lead.
Attention now turns to the remaining Alpine stags, starting with the summit finish tomorrow in Tignes. Will Ineos ride for Bernal or 2018 Tour winner Thomas? And will Alaphilippe be able to maintain his 14-day yellow jersey run?
"There are two big, big days now," Thomas said. "So obviously, we knew it would be hard to do anything to drop Alaphilippe today, but it was a big day and there's two more big days to come."
After stage 19 tomorrow, the Tour continues with his last classification test, a summit finish at Val Thorens. The race ends with a traditional sprint stage in Paris on Sunday.
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