Porte (BMC) was the only title contender that could match an acceleration from Froome (Sky) in the final 2.5km of the uphill finish in Vaujany, which the latter won to take the leader’s jersey from Alberto Contador (Tinkoff).
The pair took turns in the closing kilometres before the 31-year-old Porte faded just before the line.
The Australian immediately after the stage said he’d been advised not to collaborate with Froome after the Briton attacked, however, believed it more beneficial to do so.
“It’s a bit confusing when the team are telling me not to ride with him and, to be honest, it’s probably better to ride together to the line,” Porte said.
“I think it’s good for July that we’re both going so well. It’s little bit different to be riding for yourself, I’m quite enjoying it now and the team were really good around me today so I’m happy with how things are.”
Froome agreed the move was of dual interest with two decisive mountain stages remaining.
“It’s definitely quite different racing against him but a day like today I think it was in both our interests to ride together, as we have done in the past,” the defending race champion said. “It’s not the first time we’ve crossed the line first and second on a mountaintop finish and hopefully it won’t be the last either.”
Froome, leading into Saturday’s queen stage from La Rochette to Meribel, has a seven second advantage over Porte, with Contador 27 seconds in arrears and Dan Martin (Etixx-Quick Step) a further 10 seconds back.
“[Friday] being the first real uphill finish, it was the first opportunity to really leave everything on the road and just see exactly where everyone is in terms of condition,” he said.
“I’ve felt good since the beginning of the week. The team has ridden extremely well around me, kept me up in contention at the critical moments and they did a lot of work today coming into the final. So for me also that gives a lot of added motivation, when I see my teammates are working so hard like that.
“It’s a really nice feeling to be back in yellow and to win the stage but tomorrow is another really hard day,” he continued.
Neither Froome nor Porte have discounted Contador, who cracked on the last climb of the fifth stage. The champion Spaniard had led the tour since the 4km uphill prologue he won last Sunday.
“He was a little bit off the pace but if I’ve learnt anything from racing against Alberto [it’s] he always comes back even stronger after he’s had a bit of an off day,” Froome said. “We saw in the prologue already that he’s extremely strong and he won that quite convincingly.”
Meanwhile, Adam Yates (Orica-GreenEdge) marked an impressive performance in the 140km fifth stage from La Ravoire to Vaujany, notably finishing third behind Froome and Porte. The 23-year-old currently sits sixth overall.