Cassidy, who claimed third on Saturday's race after Nyck de Vries was handed a late penalty, posted a 1m12.933s to head the session - which withstood a late effort from da Costa who offered a glimmer of hope for DS Techeetah as the French manufacturer has struggled so far in London.
Mahindra's Oliver Rowland was the first to bring the times below the 1m20s mark, as the drivers shook off the morning cobwebs in earnest, but that preceded a flurry of push-laps that ensured the timing boards resembled a slot machine.
The slots eventually landed on #17, as de Vries - whose third place in Saturday's race became sixth after his five-second penalty - leaped to the top within the first 10 minutes.
But de Vries was briefly prised out of the headline slot, as Jake Dennis - who had dominated proceedings on Saturday - eclipsed the Dutchman by a tenth and a half.
Nonetheless, de Vries hit back, dropping underneath the 1m14s mark with a 1m13.925s - and was quickly followed by teammate Stoffel Vandoorne, who moved up to second.
But just before the halfway point of the session, Edoardo Mortara fired his Mercedes-powered Venturi into first, logging a 1m13.140s on 250kW power.
The Swiss driver reckoned after Saturday's failure to score that his points deficit to Vandoorne was "starting to be too big to think about championships", but showed improved form on Sunday morning.
His Venturi teammate Lucas di Grassi fired up to second behind him, having yesterday needed to make a recovery through the field as his qualifying laps were removed for impeding.
Dennis then returned to the top, shading Mortara by half a tenth. As the final 10 minutes began, Mortara had a half-spin at Turn 9 and came into contact with Jean-Eric Vergne, but thankfully it looked as though neither sustained much damage.
Pascal Wehrlein then moved up to second to hint at improved pace for Porsche, but Sergio Sette Camara then captured top spot by 0.062s over Dennis - but briefly as Robin Frijns surpassed the Brazilian by 0.004s.
Cassidy then made it an Envision 1-2, diving below the 1m13s with a 1m12.933s - the Kiwi yesterday benefitting from de Vries' time penalty as the FIA felt that the Mercedes driver had weaved in the braking zone for Turn 1.
But da Costa split the green machines at the close of the session, setting a 1m12.972s - just 0.04s down on Cassidy.
Frijns thus finished the day in third, with Sette Camara fourth as he hopes to rescue points for the Dragon Penske squad after falling short in Saturday's race.
Sette Camara was investigated for overpower in the session, and Dragon struggled with systems issues in the first race which meant that the team could not see how much energy each driver was consuming.
Dennis was fifth, a day after leading every single session, with Wehrlein sixth ahead of Vandoorne.
Mortara eventually ended the session in eighth, touching the wall again towards the end of the allotted time at the chicane, with Rowland and di Grassi completing the top 10.
Formula E London E-Prix - FP3 results
| Cla | Driver | Team | Laps | Time | Gap | Interval |
|---|---|---|---|---|---|---|
| 1 |  Nick Cassidy |  Virgin Racing | 19 | 1'12.933 | ||
| 2 |  Antonio Felix da Costa |  Techeetah | 17 | 1'12.972 | 0.039 | 0.039 |
| 3 |  Robin Frijns | Virgin Racing | 17 | 1'13.026 | 0.093 | 0.054 |
| 4 |  Sergio Sette Camara |  Dragon Racing | 20 | 1'13.030 | 0.097 | 0.004 |
| 5 | Jake Dennis | Andretti Autosport | 18 | 1'13.092 | 0.159 | 0.062 |
| 6 |  Pascal Wehrlein | Porsche Team | 20 | 1'13.121 | 0.188 | 0.029 |
| 7 |  Stoffel Vandoorne | Mercedes | 16 | 1'13.126 | 0.193 | 0.005 |
| 8 |  Edoardo Mortara |  Venturi | 17 | 1'13.140 | 0.207 | 0.014 |
| 9 | Oliver Rowland |  Mahindra Racing | 20 | 1'13.151 | 0.218 | 0.011 |
| 10 | Lucas di Grassi | Venturi | 20 | 1'13.171 | 0.238 | 0.020 |
| 11 | Mitch Evans | Jaguar Racing | 21 | 1'13.195 | 0.262 | 0.024 |
| 12 | Nyck de Vries | Mercedes | 16 | 1'13.202 | 0.269 | 0.007 |
| 13 | Jean-Eric Vergne | Techeetah | 19 | 1'13.202 | 0.269 | 0.000 |
| 14 | Maximilian Gunther | DAMS | 19 | 1'13.284 | 0.351 | 0.082 |
| 15 | Sam Bird | Jaguar Racing | 18 | 1'13.334 | 0.401 | 0.050 |
| 16 | Andre Lotterer | Porsche Team | 19 | 1'13.419 | 0.486 | 0.085 |
| 17 | Oliver Askew | Andretti Autosport | 16 | 1'13.420 | 0.487 | 0.001 |
| 18 |  Antonio Giovinazzi | Dragon Racing | 20 | 1'13.507 | 0.574 | 0.087 |
| 19 | Oliver Turvey | NIO Formula E Team | 19 | 1'13.692 | 0.759 | 0.185 |
| 20 | Alexander Sims | Mahindra Racing | 18 | 1'13.884 | 0.951 | 0.192 |
| 21 | Sébastien Buemi | DAMS | 18 | 1'13.939 | 1.006 | 0.055 |
| 22 | Dan Ticktum | NIO Formula E Team | 20 | 1'14.011 | 1.078 | 0.072 |
| View full results | ||||||
