Cassidy had started ninth on the grid but fell back into the pack as far as 21st come Lap 21 in an intensely competitive encounter, where three wide for the race lead was the norm.
The New Zealander came from nowhere to mount a late-race charge, having successfully navigated two spells under Safety Car conditions with energy in-hand on the lead pack, and admitted post-race he was unsure he had the car to pull off such a feat.
RESULTS: The 2024 SUN Minimeal Berlin E-Prix Round 9
Cassidy's winning move came on Lap 43. Having dispatched Oliver Rowland (Nissan) - the Yorkshireman briefly a race leader having started way down the order in 15th - the Jaguar driver then sliced by DS PENSKE's Jean-Eric Vergne later that same lap.
From there, a percentage point of usable energy to the good having raced in the pack for a good portion of the encounter, Cassidy bolted to a two-second margin, the win and the standings lead as those behind, from second to sixth, fought two-by-two and three-abreast through the hairpins for the remaining spots on the podium.
Vergne was characteristically stoic in his defence, fending off Rowland for second despite the Brit's best efforts. Mitch Evans (Jaguar TCS Racing) fought from 10th and kept his nose clean to wind up fourth, ahead of another pair of drivers that had hit the front earlier on in the Porsche duo of Pascal Wehrlein and Antonio Felix da Costa.
Stoffel Vandoorne (DS PENSKE) had started second and headed the pack on a number of occasions but could manage only seventh, ahead of Edoardo Mortara (Mahindra Racing) and Nissan's Sacha Fenestraz.
Taylor Barnard more than impressed on a frenetic full Formula E debut, becoming the youngest ever points-scorer in the series' history with a 10th-placed finish.
All that saw Cassidy top the Drivers' World Championship heading into Round 10 on Sunday, with 121 points to Wehrlein's 112. Jaguar TCS Racing lead the Teams' running 210 points to 146 from TAG Heuer Porsche - the British outfit enjoying another trip to the top step after their Monaco one-two.
Round 10 follows tomorrow, 12 May, at Tempelhof.
As it happened...
Mortara covered off Vandoorne into Turn 1 as the pack filtered through unscathed - only Sette Camara of the top six made moves, passing Vergne for third at the final corner but only temporarily.
The race leader jumped for ATTACK MODE on Lap 3 - the first of the mandatory 50kW power boosts to take in the race - which dropped him back into the pack in fifth, allowing Vandoorne to hit the front with teammate Vergne second. Sette Camara followed suit on Lap 4, while Wehrlein took the lead from the DS pair at the hairpin.
On Lap 5 Wehrlein ceded the lead to take ATTACK MODE, releasing the DS' once more - Vandoorne now leading once again from Vergne and da Costa with polesitter Mortara fourth.
Once the first round of ATTACK MODE shook out, Vergne led Vandoorne, with the DS pair working in tandem to maximise efficiency while keeping track position. The Porsche pair of Wehrlein and da Costa followed. Come Lap 9, Rowland was the big climber in the Nissan from P15 on the grid to eighth, while Nick Cassidy (Jaguar TCS Racing) was the big loser, falling from ninth to 17th.
At a quarter race distance, Vergne led Vandoorne, Wehrlein, Mortara, da Costa, and Sette Camara the top six while Joel Eriksson's Envision was recovered under a Full Course Yellow then Safety Car - the Swede clipping the wall and breaking his right-rear suspension.
On Lap 17 the race went green once again, with the lead a fluid concept between any of four cars out front as the second round of ATTACK MODE activations began. Max Guenther was the man on a mission - jumping several to take fourth spot while Vandoorne slipped to 12th. The German, though, made a costly error in missing the ATTACK MODE loop on Lap 21.
Mortara hit the front once again as the race hit Lap 22, before being passed again by early leaders Vergne and Wehrlein, with da Costa third on Lap 24. Jake Dennis, meanwhile, had clambered from the back of the pack to eighth with as much as six percentage points of energy in-hand on some of the leaders. The Brit was also complaining over the radio that he was struggling to see out of one eye. Rowland's climb continued, too, the Nissan driver now 12 spots up on his grid slot and third on Lap 25, despite front wing damage.
Less than a second split the top eight with 27 laps down. Wehrlein and da Costa were the current leaders, though things were changing by the sector.
Evans hit the front on Lap 28, though the Kiwi was the only driver with an ATTACK MODE to take.
Guenther's forward foray came to an abrupt end, as contact with Hughes on Lap 30 speared the Maserati Tipo Folgore into the wall and out of the race - requiring a second appearance for the Porsche Safety Car.
Evans led things away on Lap 34 once again, with Wehrlein holding off Rowland before the Yorkshireman fired it up the inside of the Porsche and Evans for the lead at the hairpin before da Costa sliced by just a turn later - carnage!
The new leader had more than a percent of usable energy on those immediately around him, though he would be passed by Vergne and Rowland once again on Lap 35 - again just a second between first and eighth.
Dennis ran deep into the hairpin on Lap 36, losing four spots and ultimately having to pit thanks to a right-front puncture having come into close contact with da Costa's Porsche. With six laps left and energy in-hand, having driven from 20th on the grid, it proved to be a super costly coming-together - the Andretti having run as high up the order as fifth.
Six laps were added for those spells under caution, and Evans headed the way from Vergne - having taken his remaining ATTACK MODE - with Wehrlein, Cassidy, Rowland and da Costa rounding out the top six.
Every corner seemed to be three-wide for the lead and beyond as the laps ticked away. On Lap 41, Evans lifted early at the final turn - seeing him fall into the gaggle behind while Vergne once again hit the front.
Cassidy, meanwhile, had come from absolutely nowhere - 21st on Lap 21, having slipped down the order from ninth on the grid - to take second from Rowland with just three laps left to run and hit the front, passing Vergne later that same lap (Lap 43).
From there, he bolted - the Jaguar driver some 1.5 seconds quicker than Vergne behind and striding to a two-second advantage out-front as second to eighth squabbled.
It was a free-for-all for the podium heading into the final tour. Some hard driving saw Vergne hold second from a battling Rowland, while it was two by two to the line as Evans fought to fourth, Wehrlein fifth and da Costa sixth.