The New Zealander picked up a solid advantage of 0.152s over Vandoorne in the first sector during their final duel, but Vandoorne hit back in the second part of the lap to sit 0.04s ahead.
But Cassidy refused to give up the three points rewarded for pole position and kept his foot in it during the final corner to lay down the gauntlet - and thus, Vandoorne missed out by less than a hundredth of a second.
