Sky's Welshman warmed down with a long-sleeve white Sky jersey over the Tour de France's leader's jersey, appropriate because he slipped to second overall at 12 seconds behind team-mate Chris Froome.
"It is a bit sad to take off the yellow," Thomas told Cycling Weekly. "You don't realise what you've got until it's gone. I've certainly appreciated it and enjoyed it.
"It's going to be weird and stick on the new white jersey. At least I kept it fresh. I have a fresh white jersey and the boys' are getting a bit dirty!"
Thomas pulled in the Sky train up the short 5.9-kilometre climb. Fabio Aru (Astana) attacked for the eventual win and Froome followed. Thomas faded slightly and finished at 40 seconds behind Aru and 20 behind Froome.
"To be honest, I didn't expect to keep it, and to still be relatively close is more of a surprise than not keeping it," he added. "To be second in GC still is a nice bonus. I didn't think that going into the day."
The mountain days have only just begun. The race will swing through the Jura Mountains before hitting the Pyrenees and Alps. Thomas will have his chance again.
Watch: Tour de France 2017 stage five highlights
"Tactically it could play a part [to be high in the overall] come the weekend because there could be a lot going off. It'd be nice to stay up there," Thomas said.
"My goal coming in was to be with Froomey in the thick of the action every day, every big day, and if I do that, that means I stay up on GC anyway.
“For me, anything is a bonus now. Like I said, I've been going well since March. The Giro d'Italia [abandon] was a massive hit, so to go down and to come back up to try go get what I could out of this race, stage win and four days in yellow... I could just sit at the back and enjoy life now!"
Thomas will not have a chance to do so. He must work for Froome as he did today. In the first summit finish of three today, he only touched the front briefly after Michal Kwiatkowski and Mikel Nieve.
"I felt better than expected to be honest, it was just... I was comfortable with the pace if we'd gone a bit faster, but as soon as the guys started kicking I didn't have the legs to go, but I kept trying to ride a decent tempo and came back to the front for the final corner. When they sprinted for the final steep thing, I was out the back again.
"We learned a bit as well, like I said to [Froome] at the finish, when he attacked I could've done that for him but he showed he has decent legs. We gave Aru a little bit of leeway which we won't do again.
"We didn't want to let [Aru] get as much time as he did. We kind of lost Landa and Sergio Henao, but I think they need a little bit of time to get into the race as well.
“Frosty [Nieve] was riding a decent tempo but not quite hard enough and Aru got a good advantage then. We learned a bit from that and we won't let that happen again. But for sure, Richie Porte, Aru, Dan Martin – they are the stand-out names at the moment.”
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