New Zealander Evans defended fiercely against compatriot Cassidy over the final lap, notably into the Turn 4 chicane and through the final sequence of corners, as the leading trio were covered by just over half a second at the flag.
Antonio Felix da Costa was the leading Porsche runner in fourth, heading home the DS Penske duo of Jean-Eric Vergne and Stoffel Vandoorne.
Championship leader Pascal Wehrlein remarkably finished seventh despite starting 18th, as contact between himself and title rival Jake Dennis resulted in the Andretti Autosport driver's retirement.
Polesitter Vandoorne got the perfect launch away at the start and held a comfortable lead through the opening sequence of corners, while da Costa had to repel a challenge from Evans – the Jaguar driver taking to the outside line into Turn 1 but settling back into third.
Behind, Nissan’s Norman Nato lost his front wing over the back of Jake Hughes’ McLaren as the pack slowed for the right of Turn 3, while Edoardo Mortara (Maserati MSG) also lost his front wing through the opening sequence of corners and dropped out of fourth.
While the rest of the opening lap passed without further incident, Sebastien Buemi (Envision) was another to require a trip to the pits after tripping up over the back of Maximilian Guenther’s Maserati MSG through the Turn 4 chicane on the second lap and damaging his front wing.
Out front, Vandoorne took the decision to active one-minute of attack mode on lap five, dropping him behind da Costa, but at the start of lap seven the reigning champion moved back to the front with a move into Turn 1.
Vergne and Bird were two drivers to active attack mode on lap six, with Bird taking the decision to use three minutes of his allocated four to Vergne’s one.
The extra boost allowed Bird to power past Hughes into Turn 1 at the start of lap eight, moving him into sixth behind Vergne, as Vandoorne continued to lead from Evans and Cassidy while da Costa dropped to fourth.
The condensed field was offered no chance to spread out as a safety car was called moments later to retrieve the stricken Nissan of Sacha Fenestraz, which come to a halt on the exit of Turn 6.
At the restart on lap 12, da Costa was the driver initially on the move, diving to the inside of Evans at Turn 1, and getting back ahead of Cassidy into Turn 4 moments later.
With the slipstream so powerful, both Vandoorne and da Costa backed off to avoid leading, allowing Cassidy – now up to third – to overtake them both at the start of lap 14, before dropping back to second as he activated one-minute of attack mode.
Behind, championship rivals Dennis and Wehrlein made contact into Turn 1, with the former stopping out on track and instigating a second safety car.
Replays showed he had been hit from behind by Dan Ticktum into Turn 1, which damaged his car ahead of the secondary Wehrlein contact.
Prior to the safety car on lap 16, Cassidy had moved back to the front with da Costa also demoting Vandoorne down to third as the Belgian tried to regain a deficit on energy of some 3% compared with his rivals.
As things settled down at the front Cassidy began to stretch his lead, moving to 1s ahead of Evans while both continued to pull away from Vandoorne, who was attempting to save further energy having led in the early stages.
Wehrlein had remarkably climbed from 18th to fourth by the time he used three minutes of attack mode, dropping to seventh behind Vergne, Bird and da Costa.
A lap earlier, his Porsche team-mate da Costa lost any chance of victory by running wide at Turn 1 and needing to come to a complete stop before rejoining. It dropped him from third to seventh.
In front of the Porsches, Bird moved into fourth at the expense of Vergne into Turn 1 on lap 29, and soon disposed of Vandoorne for third before setting after the leading duo.
The gap to Evans stood at nearly 2s, but this quickly reduced as Evans began to pressure Cassidy and made his move into Turn 1 at the start of lap 31, with four extra laps added due to the two safety car periods.
Cassidy took a watching brief until the final lap, and tried several times to find a way past, but had to settle for second.
