Thomas survived the last big mountains on stage 19 in the mammoth climbing day over the Tourmalet and Aubisque to finish in Laruns. He leads Tom Dumoulin (Sunweb) and at 2-24 minutes, Primož Roglič (LottoNL-Jumbo) in third.
"Surely it is enough," Thomas said of his lead. "We'll see. It should be enough but we will see."
The Welshman faces the final 31-kilometre time trial to Espelette on Saturday against Dumoulin, the current world time trial champion, and Slovenian Roglič who is a very accomplished rider against the clock.
Coming out of Friday's test with a two-minute lead, Thomas could relax a bit. He faced multiple attacks from Dumoulin, Roglič and other rivals further down the overall classification when they climbed the Bordères and Aubisque.
"I will be a bit more relaxed but I still have to get through," Thomas said.
"It was a big day today. I needed to get that ticked off. It's a bit like an Olympic final, the day of tension. Obviously it comes down to tomorrow but I can take confidence from how I am riding."
The attacks today drew out Thomas, who marked Roglič and Dumoulin when helpers Froome and Egan Bernal drifted behind. However, he kept calm under pressure.
"I had to make him [Dumoulin] do everything. It's the first time I have seen him get a bit tired," Thomas added.
"I just put the pressure on Dumoulin to close. Tom wanted to attack me. I just stuck to Tom and made him close most of the gaps and it worked really well.
"I was able to respond to the attacks and the descent was fast. I never thought I was going to get dropped but it was hard for sure."
The only loss on the stage was to Roglič, who slipped ahead on the 20-kilometre descent and gained 19 seconds at the finish line. With the 10-second bonus, he also leaped ahead of Froome to move to third overall.
Thomas and the team must now rest before the "tough" time trial near the Spanish border town of Biarritz.
"It's certainly better than being two minutes behind," Thomas said. "It's a tough course but we'll see."
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