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.
