Fellow Andretti driver Kyle Kirkwood began the final qualifying session at a disadvantage, having used two sets of fresh alternate Firestones to get out of Q1, so had no fresh alternates for the last segment.
However, that issue seemed small when he slid straight on at the final turn, and missed the tyre barrier to crunch hard into the concrete.
Once the action resumed, Penske’s Scott McLaughlin broke a right-rear toelink on the Turn 10 wall, which sent him into a pirouette through the chicane, causing another red flag.
Despite also suffering the same issue as Kirkwood in terms of having no fresh alternates, Marcus Ericsson briefly took top spot when the session finally began in earnest, but was soon deposed by Pato O’Ward’s Arrow McLaren and Colton Herta of Andretti Autosport.
Yet Grosjean then delivered a stunning final sector to beat Herta’s benchmark by 0.4155 seconds and claim his first pole position in an Andretti car.
O’Ward was just 0.0476s behind Herta and 0.42s ahead of Ericsson.
Patricio O’Ward, Arrow McLaren Chevrolet
Photo by: Jake Galstad / Motorsport Images
Earlier, Alexander Rossi sat top of the Q2 times after everyone’s first runs, with McLaughlin second but a full 0.28s behind, chased by Scott Dixon, Will Power and Herta.
Everyone then ducked in for fresh alternate ‘greens’, and Felix Rosenqvist delivered another sub one-minute effort, which was then shaded by Kirkwood.
There was disappointment for Chip Ganassi racers Alex Palou and Dixon, however, as they are set to start seventh and ninth.
Rosenqvist and Rossi were eighth and 12th for Arrow McLaren, and nine-time St. Pete polesitter Power only 10th, just ahead of Christian Lundgaard of RLL.
Two-time St. Pete winner Josef Newgarden was among the early Q1 casualties as he was unable to enjoy a truly clear lap and will therefore start only 14th, while Rinus VeeKay of Ed Carpenter Racing was inadvertently held up on a flyer by Palou.
Although he didn’t make it through, Argentine touring car ace Agustin Canapino did a commendable job in his first ever open-wheel qualifying session in the group 1 segment to wind up less than 1.2s slower than Rosenqvist’s benchmark and he will line up 21st.