The Briton finished third on-the-road in a tense penultimate round, which would have been enough for the title, but the result was guranteed when he was promoted to second after Antonio Felix da Costa was handed a three-minute penalty for a technical infringement after finishing second.
Dennis’ closest championship rival Cassidy had led early in the race from pole, but his championship chances were dashed after Envision Racing team-mate Sebastien Buemi damaged his front wing.
Cassidy made the perfect start from pole while Dennis was forced to defend from Buemi into the opening turns, but he had to give best to the Envision driver on the exit of Turn 4.
Buemi initially proceeded to act as rear-gunner to Cassidy and began to back the pack up, with Cassidy able to take both Attack Mode activations inside the first seven laps and rejoin in the lead both times.
But things began to unravel for the championship protagonist when the Kiwi let Buemi ahead into T16, a move which would allow his team-mate to take his own Attack Mode activations and lose less time and positions.
Evans – who had risen to third from sixth on the grid, having topped qualifying before serving his five-place grid penalty for colliding with Cassidy in Rome – also took the opportunity to move ahead of Cassidy into the left-hand Turn 17.
When Buemi activated his first Attack Mode on the next lap he rejoined in front of Cassidy, whose attentions turned to Dennis behind him.
The Andretti driver made an audacious move around the outside into the right-hand Turn 19, which then gave him the inside for the final left-hand bend, albeit on a very tight line.
The loss of momentum for both Dennis and Cassidy allowed McLaren’s Rene Rast to move ahead of them and into third.
Three laps later Cassidy retaliated, moving to the outside into Turn 1 and running side-by-side with Dennis before finally regaining the position into Turn 4.
Now back behind his team-mate, Cassidy looked to move back in front but Buemi took defensive lines over the next two laps, seemingly with no orders from his team.
The title was turned on its head at the start of lap 15, with Cassidy moving to the outside into Turn 1 and attempting to repeat the same overtake he had made on Dennis.
But into the Turn 4 right-hander, Buemi cut in front of his team-mate which damaged Cassidy’s front wing and meant a trip to the pits before he retired – dashing his title hopes.
A safety car was deployed to retrieve the debris left by Cassidy’s front wing, with Evans leading at the restart from Buemi, Rast and Pascal Wehrlein.
Porsche’s Wehrlein battled with Dennis, despite protests from the Andretti driver – the team a Porsche customer outfit – who felt that he should be allowed ahead.
Dennis’ struggles became even worse as he missed a second Attack Mode Activation and dropped to sixth. But he was soon promoted up the order at Wehrlein’s expense, after the Porsche driver had come to grief into Turn 1 as Rast made contact with him.
The race was dramatically halted with just four laps remaining after Sacha Fenestraz made contact with the back of NIO 333’s Sergio Sette Camara into the braking zone of T16.
The impact launched Fenestraz’s car briefly into the air, with the Nissan ploughing into the barrier in the run-off area and noticeable moving the barriers backwards. The red-flag was deployed to repair the barrier, with Fenestraz uninjured, the race for the championship becoming essentially a five-lap dash, with an extra lap added due to the safety car periods.
Dennis’ charge was helped further when Rast was forced to start at the back, after his front wing was replaced under the red flag.
Both Evans and Buemi took their remaining Attack Mode activations at the first opportunity on the rolling start, Evans rejoining back in the lead from da Costa and Dennis, with Buemi dropping to fourth.
The Swiss then inexplicably slowed down by several seconds over the next lap and had dropped off the back of the top three.
Norman Nato attempted a move into T19, but Buemi turned in on the Nissan driver with the closely following Sam Bird also making contact. This blocked the track for the remaining drivers behind, prompting a second red flag.
The order remained unchanged on-the-road over the final two laps, Evans claiming the win with Dennis promoted to second at da Costa’s expense as Buemi completed the podium.
Bird, minus front wing following the crash at T19, finished fourth with Sette Camara completing the top five.
