Alexandre, who asked AFP that his surname be withheld, was among the crowd that massed on Nice's stately palm-lined Promenade des Anglais for the spectacular Bastille Day fireworks display last Thursday.
"I came across the truck as I was leaving the fireworks on my bike," he told AFP.
"At one point I told myself I would cross the promenade to take the bicycle path along by the sea to go faster.
Mohamed Lahouaiej Bouhlel, also 31, had already left a trail of devastation, zigzagging down the street at speed to try crush as many people as possible with his 19-tonne refrigerated vehicle.
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Alexandre caught onto the handle of the cab door on the driver's side to try to stop him.
The Tunisian father-of-three at the wheel clapped eyes on him and then began searching for something in the truck.
"It was his gun he was looking for," Alexandre said.
Grabbing a hold of his pistol Bouhlel pointed it at the young crane driver, who promptly let go of the handle.
At that moment he felt a motorbike brush past him as the other hero of the night, whose identity is still unknown, entered the fray.
The motorcyclist chased the truck and also tried to open the driver's door to overpower Bouhlel but fell off and ended up under the wheels, according to a German journalist who filmed the events on his phone.
It is not known whether the motorcyclist survived.
Bouhlel, a delivery driver by profession, looked to be "in full control of his vehicle" as he careened down the street, Alexandre said.
By the time police deployed to guard the July 14 festivities had caught up with him and shot him dead, Bouhlel had crushed 84 people to death, including around a dozen children, and injured around 300 others.
Tens of thousands of people marked a minute's silence for the victims on the seaside promenade yesterday.