The Andretti driver was never seriously headed after taking both his Attack Mode activations in the first section of the race, having taken the lead from polesitter Vergne in the process.
Vergne was able to hang onto second but only just after suffering from a severe lack of energy in the final laps, with Mitch Evans attempting a pass on the final lap which ultimately failed and dropped him off the podium.
Vergne held the lead from pole and pulled a small gap as Evans was forced to give space to Sergio Sette Camara, who from fourth on the grid had sent his ERT up the inside on the opening turn.
Evans maintained second, while the loss of momentum for Sette Camara allowed Dennis to move back into third after getting a poor getaway off the line.
On lap three, and at the earliest opportunity, Vergne took the first of his two Attack Mode activations, losing the lead and dropping behind Evans and Dennis.
Evans relinquished the lead a lap later, but on rejoining from his own Attack Mode activation made side-to-side contact with Vergne as he rejoined but fell back into third.
The loss of momentum for the pair allowed Dennis to pull a small gap in the lead and when the Andretti driver took his Attack Mode on lap six, he rejoined in second, behind Vergne but ahead of Evans.
Source: Autosport