Having taken his maiden pole position at the Cape Town E-Prix after beating Maximilian Guenther in their qualifying final, Fenestraz had lost out to the eventual top two of Antonio Felix da Costa and Jean-Eric Vergne over the course of the 32-lap race.
The Franco-Argentine had managed to put a move on long-time leader Cassidy to move up to third, which he looked set to hold onto until the final lap when Fenestraz - unseen on TV - hit the wall and tumbled down the order.
Cassidy reclaimed the final step on the podium as a result, with Fenestraz classified last.
Explaining the incident, Fenestraz felt that Cassidy made contact with the rear of his Nissan with his front wing, bringing an end to any aspirations of a first Formula E podium.
"We were racing for the third place of the race and it seemed like he was racing for the championship," Fenestraz told Autosport.
"It's a bit of a shame - I mean, some people thought I ended up in the wall by myself, but wasn't like that.
"He didn't even lunge really at Turn 7, he had a slight bit of the front wing behind my rear wheel, but he was nowhere near side-by-side.
"He just touched me and then I lost the rear straight away and I just hit the Tecpro barrier. So it wasn't a big contact, but big enough to break my rear wishbone.
"It's a shame because you know, we were on for a good position. A move like that from Cass... yeah, it's a shame."
Cassidy disagreed with Fenestraz's point of view, and suggested that the incident was caused by the French-born driver moving under braking having left the door open.
The New Zealander contended that Fenestraz's defence was as a result of him struggling with energy, which Cassidy had hoped to pounce on.
"He was just basically short on energy, so he had to defend pretty hard. But he left the door really open at Turn 7, which is a great passing spot.
"And then when he saw me go for it, he's covered, which we've spoken a lot about - moving under breaking and double movements - in the past.
"He's done that and come across my front wing. I'm not really a fan of that moving under braking. It's been talked about, like I said, a lot.
"But at the same time, I feel sorry for him because the rest of the race he drove fantastically and obviously he did a great lap in qualifying. He's proven he is a fantastic driver."
Fenestraz was involved in an earlier contretemps with Maximilian Guenther during the early full-course yellow, where the Maserati MSG driver passed him for the lead as the yellows were shown and did not relinquish the place.
Guenther, having retired at the time he was handed a penalty from hitting the wall, has been handed a three-place grid penalty for the following Sao Paulo round.
