Frenchman Alaphilippe extended his overall race lead with a barnstorming performance, 100 years to the day the coveted golden fleece was first worn by Eugene Christophe in 1919.
"I knew that I could do a good performance on this parcours," he said of his second stage win at this year's Tour de France.
"I just gave everything. Especially the first part, it was really good for me and I just went full gas. I just see what I can do until the line. I can't believe it. With a parcours like this and my shape, I just did everything I can. I just wanted to enjoy."
Increasing attention is being paid on whether Alaphilippe can hold the yellow jersey past the Pyrenees or even into Paris with his current form.
"To do something like I do now, with the confidence and my shape and the team around me... okay we don't have the team to win the Tour de France," he said. "We don't have a lot of climbers in Deceuninck-QuickStep for the Tour but we ride aggressive and try to win. I'm just so happy at what's happened now."
Perfect conditions greeted the riders for a hilly course around traditional Tour de France stage town Pau.
Kasper Asgreen (Deceuninck-QuickStep) set the early pace, with the Danish domestique posting an impressive time of 35'52 that stood until Thomas De Gendt's arrival.
De Gendt (Soudal Lotto) bettered Asgreen's time by 16 seconds, moving him into the hot-seat to see who might go faster over the 27.2 kilometre course.
Pre-race favourite Wout van Aert (Jumbo-Visma) hit out hard early, setting the fastest time through the first checkpoint before coming down heavily in a crash, his handlebars hooking into the barriers as he came round a corner at high speed. Onlookers scrambled to help the Belgian star, but van Aert was forced to abandon.
Porte (Trek-Segafredo) set a blistering pace from the outset, sitting just behind De Gendt's impressive benchmarks at the early time checks. He conceded time in the technical run to the finish where a number of riders crashed earlier but crossed the line second provisionally with his major GC rivals still to come.
Porte finished fifth, 45 seconds behind Alaphilippe, but taking significant time on a number of his closest opponents.
"It was a hot one," said Porte. "I felt good. Obviously, having a crash a couple of days ago there has knocked me about a bit. I'm happy with the time, I know there's still a few strong guys to come. We'll see how it stands up. The climbing part was good for me but the last bit of flat I really suffered."
"The team's good, they're behind me. There's no stress. Obviously, it's not been ideal two stages there, but the rest of the first ten days have been really good. Next couple of days are a real test and everything is going well."
Rigoberto Uran (EF Education First) pushed De Gendt's mark desperately close, but it was Geraint Thomas (Ineos) that blasted past to set the fastest time, only bettered by Alaphilippe shortly afterwards.
The Frenchman skidded to a halt in front of his team staff to begin the celebrations in earnest as he not only defended his yellow jersey but increased his lead over every other rider in the field.