The Porsche driver decisively took the lead of the race at the start of lap 21 following a brief safety car to retrieve debris and managed his remaining energy over the remaining six laps to secure his second win of the weekend at the Portland International Raceway.
Having been gifted Saturday’s win after a five-second penalty was handed to on-the-road winner Mitch Evans, there was no doubt of da Costa’s win on Sunday despite late pressure from Robin Frijns.
Evans completed the podium to move to within just 12 points of Jaguar team-mate Cassidy, who after spinning out of the lead in Saturday’s race, failed to score again on Sunday after pitting for a new front wing.
Fellow title protagonist Pascal Wehrlein finished fourth minus his front wing after contact with Edoardo Mortara earlier in proceedings, and also now sits 12 points behind Cassidy in the standings.
The Porsche driver’s front wing became lodged under his car following the contact with Mortara before flying off and into the path of Sam Bird, with the McLaren driver then taking to the grass along the back straight – and the incident was noted by the stewards.
But with the offending bodywork gone, Wehrlein proceeded to carry on and took vital points in his hunt for a maiden Formula E title.
His margin to Cassidy was reduced after the Jaguar driver was forced to pit following a collision into Turn 1 on lap 13 that involved reigning champion Jake Dennis and Caio Collet, all of whom suffered damage to their front wings in the concertina affect.
Da Costa also suffered damage to his front wing on lap 15 after contact with Mortara into Turn 10 at the end of the back straight, which punctured the Mahindra’s left-rear tyre and sent Mortara off.
The incident was investigated by the stewards but no further action was taken as da Costa continued with the damage.
With debris littered around the circuit the safety car was eventually deployed on lap 18, offering a lifeline to the likes of Cassidy and Dennis, who were able to catch up with the back of the pack.
After just two laps under caution, racing resumed with Frijns at the head of the field followed by da Costa, Wehrlein and polesitter Jean-Eric Vergne.
Evans occupied fifth but with still one Attack Mode left to take was always going to be at a disadvantage as the pace of the race once racing resumed ramped up considerably.
As da Costa immediately took a lead he wasn’t to lose into Turn 1 at the restart, Evans took his second activation and dropped to seventh before charging back through the pack.
He demoted Wehrlein for third into Turn 1 on lap 23 and repeated the same move on Frijns a tour later via the help of his extra boost of power.
But with that depleted he was unable to challenge da Costa over the remaining laps and fell into the clutches of Frijns, the Envision driver moving back into second into Turn 1 on the penultimate lap and with it securing back-to-back podiums in America this weekend.
Despite having considerably more energy than those ahead, the increased pace of the race after the safety car meant Cassidy was never able to utilise his advantage and could only climb to 13th.
It means Cassidy leaves the Portland double-header pointless and with a significantly reduced margin with just two races in London remaining this season.