Dennis led away from pole and just about kept himself out of the absolute disaster that struck his closest title rivals Nick Cassidy (Envision Racing) and Mitch Evans (Jaguar TCS Racing) behind on just the second lap of the race.
As the former took a look at Dennis for the lead into the braking zone at the infamous Turn 7, Evans lost the rear of his Jaguar in the compression, clipped the leader's Andretti and spectacularly launched over the top of Cassidy's Envision Racing Jaguar I-TYPE 6. Another huge moment in the battle for the Drivers' title and one Dennis took full advantage of. Evans' damage proved terminal while Cassidy could only recover to 14th come the end of the race.
From there, the Brit looked assured and scampered away to the tune of three seconds at the chequered flag - helped by Nissan's Norman Nato managing to hold off Sam Bird (Jaguar TCS Racing) for the duration despite suffering early damage to his front wing.
That was the first lights-to-flag victory of the GEN3 era, and the first Grand Slam of Julius Baer Pole Position, TAG Heuer Fastest Lap and the race win - plus every lap led. Dennis is one of only three drivers to have sealed a Grand Slam and is the first driver to have achieved the feat twice in Formula E history.
More importantly, that result means Dennis takes a 24-point advantage over Cassidy into the season finale double-header in London - home soil and a circuit he's strong at as a two-time winner. Evans is 44 points back in third, while TAG Heuer Porsche's Pascal Wehrlein retains a slim mathematical hope, 49 points shy of top spot with 58 points available.
Envision Racing head the way in the Teams' table by 13 points over TAG Heuer Porsche.
Season 9 of the ABB FIA Formula E World Championship will reach its climax in London, with two races on 29 & 30 July.
As it happened...
The top four of Dennis, Cassidy, Nato, Evans, Bird and Guenther ran as you were through the opening turns though Nato, after fending off Evans into Turn 4, made it into second spot by Cassidy. The Kiwi wouldn't let it lie though, making it around the outside of the penultimate turn. Evans followed past the Nissan on Lap 2 - not letting his title rival make a break.
On the run into Turn 7 absolute disaster struck for Cassidy and Evans. As the former took a look at Dennis for the lead into the braking zone, Evans lost the rear of his Jaguar in the dip, clipped leader Dennis and launched over the top of Cassidy's Envision Racing Jaguar I-TYPE 6. Another huge moment in the battle for the Drivers' title.
Lap 4 saw the debris cleared and the Safety Car return to the pits as the race went green again. Dennis led Nato, Bird, Ticktum, Mortara, Buemi, Guenther, da Costa, Vergne and Wehrlein the top 10. While Evans made it back out on track, he told the team over the radio that the car was "undriveable" and duly pulled into the pits. Cassidy rejoined 18th after the incident.
On Lap 6, Nato made a late move on Dennis into the penultimate turn; the Frenchman damaging his front wing and almost taking race and standings leader Dennis out. Bird, in third, looked the strongest on energy - some two percent to the good on the pair ahead.
Dennis and Nato jumped for the first of their two mandatory 50kW ATTACK MODE boosts - the Brit retaining his lead while Nato slipped to third. A trip down the Turn 7 escape road for Antonio Felix da Costa (TAG Heuer Porsche) saw the Portuguese slip down the order, promoting Cassidy to 12th as the Kiwi tried to recover. Two laps later and the Envision driver found himself 10th an in the points - looking to push past reigning champion Stoffel Vandoorne for ninth at the Marconi Hairpin.
Ticktum, meanwhile, made his initial run through the ATTACK MODE loop, which saw him shuffled behind Mortara and Guenther in the Maserati MSG Racing machines into seventh spot - the Trident's pair making steady ground.
A lap later on Lap 13, Bird also blinked - handing the lead back to Dennis, at least temporarily. He had energy in hand but now found himself behind the damaged Nissan of Nato and Buemi's Envision, in fourth place. Dennis and Nato managed to remain the lead duo despite taking their second ATTACK MODE boosts, though Bird had clambered to third beyond Buemi come Lap 16 he was now getting held up by Nato, allowing Dennis to steal the initiative in P1.
Cassidy's forward charge took a step back when he leapt for his second ATTACK MODE activation, however, with the Kiwi slipping to 15th.
The two Maseratis made a tandem move for their final 50kW boosts - Mortara and Guenther holding fifth and sixth in the process. Bird was over the radio to his time emploring them to have Envision's Buemi hold the charging blue cars up so he could take his second ATTACK MODE and stay third.
Lap 20, and Dennis looked comfortable out-front, with Nato fending off Bird - and that's the way it finished at the line with the Jaguar TCS Racing driver unable to squeeze by across the final five laps.