Cassidy, who found himself on the podium last time out in Hyderabad, was 0.307s quicker than the next man, Maserati MSG Racing's Edoardo Mortara and some 1.6 seconds faster than the pace set by Mortara in yesterday's opening sessions.
Cassidy's FP2 average speed was 154.37 km/h - the all-time fastest Formula E lap. Before this weekend, the fastest we'd seen was in Valencia, Season 7, where the average speed over a lap crossed the 140 km/h mark.
The South African street circuit has so far been a good place for Mortara, who found himself fastest during Friday's practice session, too. His Maserati teammate, Maximilian Guenther, kept himself in the top ten times, finishing FP2 in eighth.
NIO 333's Dan Ticktum rounded up the top three with an impressive lap time. Both Jaguar TCS Racing cars also looked rapid, with Mitch Evans besting teammate Sam Bird as the pair rounded in fourth and fifth. However, Bird goes into today's race with a five-place grid penalty for that incident at the hairpin in Hyderabad, which took both him and his teammate out of the race.
DS PENSKE had a mixed session, with reigning champion Stoffel Vandoone in sixth, half a second off Cassidy's time. Yet, Hyderabad race winner, Jean-Eric Vergne, could only manage 11th, just missing out on a top 10 spot.
Both TAG Heuer Porsche X99 Electric cars looked quick. However, a problem for Antonio Felix da Costa impacted the end of the session as he stopped at the chicane. He still managed to finish the session in seventh, with his teammate and current championship leader, Pascal Wehrlein, in ninth.
Envision Racing's Sebastien Buemi was back on track after an almighty effort from his side of the garage. The Season 2 Formula E champ had a high-speed clash with the wall yesterday during the first practice session, destroying the right side of his Envision machine.
Qualifying follows at 11:40 and the Cape Town E-Prix Round 5 at 16:00.