Porte crossed the finish line at the summit of the Col de La Loze in fifth place, jumping two spots overall from sixth to fourth spot, now three minutes and five seconds behind leader Primož Roglič, who also had a strong stage to tighten his grip on the yellow jersey.
The Tasmanian is now one minute and thirty-nine seconds off the current podium placers, and with the hardest stage over and done with now has every chance to make up the time and be one of the top three in Paris.
Porte admitted the stage was a particularly testing one and was happy he was able to perform in the high altitude, with thin air making it even tougher for the riders.
"It was an absolute brutal stage," Porte said. "One of the hardest I think I've ever done in my career. But to be up there in the final is nice, high altitude is not really my thing. I could really feel it in the last 500 metres, my legs really gave out."
"Tonight's all about recovering. Hopefully tomorrow it's a really controlled race."
The Australian compared the climbs on today's Col de La Madeleine and Col de La Loze to the most gruelling climb in Italy, the 1,750 metre high Monte Zoncolan, recognised as one of the hardest in professional cycling.
"To be honest, it felt like the Zoncolan," Porte said. "Which is probably the hardest climb I've done in world cycling to be fair."
"It almost felt like it would have been quicker to get off and walk up the last 100 metres!"
"It was a brutal stage, but it's the same for everybody."
Porte also lauded the efforts of teammate Mads Pedersen, who rode hard on the climbs throughout the day to stay with the Tasmanian and ensure he was in the best position for the finish.
"I mean, hats off to Mads," Porte said. "He was there with such a small group over the (Col de La) Madeleine. He was absolutely incredible, he's a real fighter and not just because he's the world champion, he's an exceptional teammate.
"He was brilliant today."
The 2020 Tour de France continues with Stage 18, last drinks for the contenders before the final individual time trial. On this 167km Alpine stage, riders face 4,000m of vertical gain. Watch Stage 18 via the SBS ŠKODA Tour Tracker from 7:55pm (AEST) on Thursday, 8:05pm (AEST) on SBS On Demand, or tune into the television broadcast from 9:30pm (AEST) on SBS.
