Cassidy and Jaguar looked to have sewn things up in a strategic, all-action encounter at the Portland International Raceway with the standings leader hitting the front for what appeared to be the decisive time on Lap 23, with four laps to run.
The New Zealander had the luxury of being able to count on his compatriot and teammate Mitch Evans to play the perfect team game despite a five-second penalty for causing a collision early on effectively rendering his own battle for silverware futile. Duly, Evans swept by da Costa - Cassidy's nearest challenger on-track as the laps ticked down - to play rear-gunner. A Jaguar one-two looked nailed on.
RESULTS: The full 2024 Hankook E-Prix Round 13 classification
Cassidy appeared in complete control and pulled out half-a-second on Evans and da Costa behind but in Formula E, nothing is certain until the chequered flag is flown. Cassidy - usually peerlessly cool and collected - slipped wide and deep into Turn 11, taking too much kerb, which saw his I-TYPE 6 fall off the asphalt, across the grass and down to an eventual 19th position.
He had won here 12 months ago, having started 10th on the grid then as he did for Round 13 but exclaimed prophetically over Team Radio mid-way through the race that he was finding the strategic battle much more difficult this time around.
Three wins from four and back-to-back victories, then, for da Costa in what was the fastest race in Formula E history, at an average speed of 152.041 km/h (94.474 mph) and the Portugese is now undoubtedly the form man. Were it not for a near-pointless opening quarter of the season and that Misano ruling, the Season 6 champion's Season 10 could have looked very different.
Jean-Eric Vergne (DS PENSKE) brought it home third ahead of Mahindra Racing's Edoardo Mortara - fourth a season-best result for both team and driver. Nico Mueller (ABT CUPRA) impressed with a race-long battle at the front and an eventual fifth-placed finish ahead of reigning champion Jake Dennis, on home soil for Andretti.
Sam Bird scrabbled through the pack from 19th at the outset to a creditable seventh spot, while Evans' penalty saw him eighth at the flag ahead of DS PENSKE's Stoffel Vandoorne and Pascal Wehrlein - the Porsche driver receiving a huge reprieve for an otherwise disappointing Round 13 with Cassidy's costly late error.
All that saw Cassidy's standings lead cut by Wehrlein to 24 points, while Evans sits a further three back in third. TAG Heuer Porsche made up ground in the Teams' running with Jaguar TCS Racing's cut to 55 points. Porsche head the way in the Manufacturers' Trophy from Jaguar 370 points to 354.
Round 14 of Season 10 follows in Portland on Sunday 30 June, at 2pm local time.
As it happened...
Evans led the pack away with Hughes challenging through Turn 2 - the Brit sweeping inside at Turn 3 for the early lead as the field fanned out four and five abreast. The two TAG Heuer Porsches ran side by side through the second half of the opening lap to hold third and fourth spots - Wehrlein from da Costa.
Come Lap 2, the Andretti pair of Nato and Dennis hit the front, clearly looking to work together early on and Lap 3 saw Nato and Hughes, the then-lead pair jump for the first of thier two mandatory 50kW ATTACK MODE power boosts, along with reigning champion Dennis amid the front-runners - the Brit having clambered from P9 on the grid to third.
The strategic battle began to play out immediately, with drivers unwilling to head the way and lifting early into the big stops - energy was predicted to be a priority for the teams on the ultra-quick, sweeping circuit.
Positions and race leaders changed corner by corner at the Portland International Raceway as groups of cars ran five and six wide at points around the wide, sweeping and ultra-quick circuit.
The order in the opening stages would hardly be representative of the final shakeup with plenty of twists and turns in the tactical balance between outright pace and remaining usable energy constantly in flux.
Hughes slipped on Lap 6, down to 21st after that strong opening five laps, while fending off Dennis. The NEOM McLaren driver took the outside line into the Esses, where a snap of oversteer on the dirty line saw him take a trip across the grass and down the order.
On Lap 7 Dennis hit the front, before taking ATTACK MODE number two a lap later. That dropped Dennis into teammate Nato as the pair continued their well-rehearsed team plan. That left the Porsche of da Costa out-front from Frijns, Dennis, Evans, Mueller, Nato, Fenestraz, di Grassi, Wehrlein and Ticktum.
Lap 10 saw da Costa into ATTACK, with Mueller. The pair led, then ceded P1 and P2 to Cassidy and Wehrlein as the pack ran six wide down the start/finish straight. Wehrlein finally made his first jump through the ATTACK MODE loop on Lap 12, which saw him shuffled from first to seventh but firmly in that lead pack - with just two seconds splitting the top 10 runners.
Sam Bird (NEOM McLaren) qualified a lowly 19th but found himself in the top three come Lap 13 and the half-way stage, just shy of leader Evans and ABT CUPRA's Nico Mueller. The Brit looked to have made the right call biding his time to climb through the pack later in the encounter.
The lead changed multiple times a lap with Edoardo Mortara (Mahindra Racing) the latest to hit top spot on Lap 15. It looked as if any driver was in with a chance of silverware.
A five-second penalty for Evans would prove a severe blow to his title charge on Lap 16 - the Jaguar driver adjudged to have caused a collision with Hughes by the race stewards. Evans found himself comfortably inside the top 10 but with the pack so close, the most he could hope for would now be a finish well down the field owards the very back of the order.
With 10 laps to run, Vergne swept all the way around then-leader Cassidy and second-placed Frijns into Turn 1 - a typically brave, bold move from Formula E's only double champion. With positions still changing by the sector, JEV's lead was short lived and Cassidy once again hit the front heading into Lap 19, making use of his second ATTACK MODE.
The "go moment" looked to have arrived with seven laps to run as the pace picked up for the sprint to the flag. "Let's play the game" said leader Cassidy, who also said over Team Radio that he was finding things much more difficult than he had during his race win here a year ago.
The Kiwi had one of the form men in Antonio Felix da Costa (TAG Heuer Porsche) in close company, with teammate Evans - carrying that five-second penalty, remember - third. Cassidy's closest rival in the Drivers' table, Wehrlein, found himself 12th and with work to do, a second ATTACK MODE activated and in-use.
The energy picture looked even out front with Frijns a percent in-hand and Wehrlein a percent down on Cassidy and the leaders.
Da Costa hit the front on Lap 22, pinching P1 from Cassidy, albeit briefly as the New Zealander took the spot back in Sector 2 the following tour. Evans played the perfect teammate despite his penalty and took the heat off leader Cassidy by slicing by da Costa's Porsche out of Turn 2 with a brave move. He could now play rear gunner.
In dramatic style, with everything looked to be sewn up, Cassidy frittered away the win, slipping wide and off the circuit at Turn 11 and all the way down to 18th - a massive reprieve to his title rivals. Nothing is ever certain in Formula E.
Da Costa profited with the win, from Frijns and Vergne following the Porsche home at the chequered flag.