But the pair were knocked off their bikes near the finish and Froome began running up the hill. He grabbed a spare bike from neutral assistance before taking back his own, but was shaking his head furiously as he crossed the line.
Thomas De Gendt won the day's stage for Lotto-Soudal from the breakaway.
After Froome finished around one minute and 40 seconds behind his rivals, 23-year-old Briton Adam Yates was shown as the new leader of the provisional general classification.
Froome began the day 28 seconds ahead of Yates in the overall standings, and was looking good to lead by more than a minute as he and Porte raced towards the finish line at Chalet Reynard - midway up Mont Ventoux with gale-force winds at the summit forcing the stage to be shortened by six kilometres.
Froome and Porte had been joined by Trek-Segafredo's Bauke Mollema and the trio were leading their rivals by around 35 seconds when Porte collided with a television bike which had been stopped by the huge Bastille Day crowds on the mountain.
All three riders hit the deck, and while Porte and Mollema got back on, Froome was left with a broken bike and, almost in a panic, he set off running up the road.
He took a bike from neutral service but soon handed it back as he struggled to get any traction, and took back his own repaired bike for the final few metres.
The provisional standings now show Yates in yellow, nine seconds ahead of Mollema, with Nairo Quintana third 14 seconds back.
Froome has fallen to sixth, 53 seconds behind Yates.
However, the race jury will have plenty of work to do before the day's results are finalised.
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