Avalanche Andretti driver Dennis had claimed a second successive pole at the ExCeL centre, as he sought to double up on victories at his home London round.
But he faced stiff competition in di Grassi, who ultimately had the upper hand in pace to make a play for the lead through the final attack mode phase - and left Dennis unable to respond in the closing third of the race.
Dennis felt that the race was "relentless" as, no sooner as he stopped fighting with di Grassi, he then had to keep Nyck de Vries at bay to hang onto second place.
"I think that was really one of the most challenging races I've ever done in my career," said Dennis.
"I thought yesterday was hard, but honestly when you've got Lucas behind you, probably he was generally quicker today.
"It was relentless, the pace at the front - and then when I tried to push, he just came with me.
"It was a demanding race on the tyres, the drivers and the car. The kerbs aren't so easy, but nevertheless, P2 is a deserved result."
Dennis explained that he made an error with one of his attack mode activations, going off-line to take it when he was already in the 250kW power mode and thus lost a smidgen of time.
He explained that it was enough to help di Grassi close in, but also felt that his tyres were struggling enough by the end of the race to ensure the Venturi driver would have had a decisive edge anyway.
The Brit also explained that it was important that he could "do my own races", and that the prestige of wins and poles were more important than the points given he does not factor in the title fight.
"Generally just made a mistake on my behalf, taking attack mode when I still have attack. So we lost 1.5 seconds for literally no reason.
"Then that was enough for Lucas to go from being out of my window to in it. So yeah, big mistake for me, ultimately it would have been hard to keep him behind the end of the race because my tyres were shot.
"Nevertheless, we've had a first and second, pole, and an incredible amount of points. It's just great to do my own races, it's more the biggest thing.
"These points for me are somewhat, yeah they're okay, but it's really the wins and the podiums that mean everything for me right now as I'm not really in the title fight.
"So yeah, 25 points is okay, but it's really the win which we'll care about."
