Di Grassi held off Jean-Eric Vergne to win the opening part of the electric single-seater series’ Canadian double-header and usurp Buemi at the top of the points.
Buemi was denied a shocking podium by the robust defence of Stephane Sarrazin on the final lap, but he will be relieved to minimise the damage to his title bid after starting 12th.
After his grid penalty for a battery change, triggered by a heavy practice shunt, Buemi was always at risk of midfield aggravation at the start.
He fell to 17th on an awful opening lap, picking up mild steering damage as a very conservative approach meant he got swallowed up in the midfield.
Buemi adapted the positioning of his left hand on the wheel to counter the offset steering and gradually regrouped, rising to a point-paying 10th by the end of the opening stint, which came prematurely on lap 15 of 35 thanks to a full-course yellow caused by the crashed Mahindra of Nick Heidfeld.
Buemi was tucked up behind di Grassi’s team-mate Daniel Abt coming into the pits, nudging the German’s rear end and gesticulating because he felt the German was deliberately holding him up.
He was released side-by-side with Abt after swapping cars, and got ahead of the German before receiving a punt up the rear after Buemi appeared to slow suddenly at the pitlane exit.
Buemi complained furiously over the radio, but ninth was his and he gained another position immediately when Adam Carroll dropped out of the points after inexplicably waiting to pit until the end of the full-course yellow.
Driving in a half-liveried second car, a legacy of the practice crash also forcing his Renault e.dams team into a barely-completed rebuild around a new tub, Buemi then caught and passed Oliver Turvey and Mitch Evans to run sixth heading into the final third of the race.
Jose Maria Lopez did Buemi a big favour when he crashed into the barrier at the final corner, sparking a safety car.
When the racing resumed Buemi’s team-mate Nico Prost moved aside without fuss, gifting Buemi fifth, and fourth was handed to him on a plate as well when Felix Rosenqvist hit the wall exiting the final corner and picked up damage to his left rear.
Despite catching Sarrazin and launching a final-lap assault, which included wheel-banging and side-by-side squabbling for three corners, Buemi was forced to settle for fourth.
The safety car that gave Buemi his late boost also wiped out a near-five-second advantage for di Grassi at the front, who suddenly had Vergne in his mirrors.
Vergne had got ahead of Rosenqvist in the car swaps, thanks to a tardy stop for the Mahindra, and passed Techeetah team-mate Sarrazin shortly before the safety car emerged.
He hounded di Grassi to the flag, but was never close enough to make a serious move.
Behind the top four was Abt, a charging Sam Bird – from 18th – and the muted Prost.
Evans, Robin Frijns and the crabbing Rosenqvist completed the points finishers.