Marcus Ericsson has won the St Petersburg IndyCar race after Scott McLaughlin and Romain Grosjean clashed while battling for the lead in the final stint.

On an afternoon of attrition, McLaughlin exited the pits ahead of Grosjean after what should have been his final pit stop, but immediately came under attack when they arrived at Turn 4.

The Team Penske driver had to go for the inside line to defend but when he pinched the rear brakes, contact resulted and both ended up in the tyre barrier.

It put the #28 Andretti Autosport Honda out of the race while McLaughlin would be slapped with a drive-through penalty after he resumed.

By then, O’Ward led comfortably but he lost first position with just three laps to go thanks to a very slow exit off the final corner of the St Petersburg street circuit.

Instead, it was Ericsson who was first to the chequered flag in the #8 Chip Ganassi Racing Honda, with a margin of 2.4113s over the #5 McLaren Chevrolet of O’Ward after 100 laps.

Scott Dixon (#9 Honda) making it two Ganassi entries on the podium, from Alexander Rossi (#7 McLaren Chevrolet), Callum Ilott (#77 Juncos Hollinger Racing Chevrolet), Graham Rahal (#15 Rahal Letterman Lanigan Racing Honda), Will Power (#12 Penske Chevrolet), Alex Palou (#10 Ganassi Honda), Christian Lundgaard (#45 RLLR Honda), and David Malukas (#18 Dale Coyne Racing w/ HMD Honda).

Grosjean had qualified on pole position and retained the lead when he took the start, but green flag conditions lasted less than half a lap.

Contact between Dixon and Felix Rosenqvist (#6 McLaren Chevrolet) caused the latter to glance the wall through Turn 3 and that checked up the back of the pack, setting off an ugly incident.

Devlin DeFrancesco’s #29 Andretti Honda got airborne when it was hit by Benjamin Pedersen’s #55 AJ Foyt Racing Chevrolet after Meyer Shank Racing team-mates Helio Castroneves (#06 Honda) and Simon Pagenaud (#60 Honda) came to grief, and Santino Ferrucci (#14 AJ Foyt Chevrolet) stopped with damage in the Turn 4 run-off area.

All escaped major injury while Sting Ray Robb drove a battered #51 Dale Coyne Racing w/ RWR Honda back to the pits and would head back out after a red flag period of more than 20 minutes.

Grosjean continued to lead when the race restarted despite pressure from team-mate and fellow front row starter Colton Herta (#26 Honda), with O’Ward third from Ericsson, Palou, and Kyle Kirkwood (#27 Andretti Honda).

McLaughlin (#3 Chevrolet) was first of those who started on primary tyres in seventh position, as did Dixon in eighth and Power in ninth, with Lundgaard 10th on alternates.

However, that was when Herta started to battle for tyre life, with O’Ward passing the #26 Honda for second on Lap 25 at Turn 10, by which point the #28 Honda was 7.7s up the road.

Herta had slumped to ninth and Kirkwood to 14th when both pitted on Lap 28 for black tyres, before McLaughlin passed Palou for fourth on Lap 29.

As the stops continued, O’Ward pitted from 5.2s behind Grosjean on Lap 31 and the Frenchman was called in on the following lap, emerging about one second up on Car #5 in the battle for the effective lead.

A significant moment occurred at the start of Lap 36 when McLaughlin pitted from the official race lead and managed to rejoin just ahead of Grosjean.

The Andretti driver looked to the outside at Turn 4 but McLaughlin got the elbows out and they touched, with the Penske driver clinging on to the position.

When Kirkwood tagged Conor Daly (#20 Ed Carpenter Racing Chevrolet) at Turn 9, a Caution was called on Lap 37, just as Dixon was taking his first stop.

The six-time champion initially retained first position behind the Pace Car but had not reached the pit lane blend line in time and would in fact be relegated to fifth under yellow flag.

It meant McLaughlin on greens led Grosjean on blacks, from O’Ward (blacks), Ericsson (blacks), Dixon (greens), Power (greens), Herta (blacks), Rossi (greens), Palou (blacks), and Rahal (greens).

Restart 2 came on Lap 42, as did Caution 3, for another wild incident.

Josef Newgarden (#2 Penske Chevrolet) lunged Rinus VeeKay for 12th position at Turn 4, with the #21 ECR Chevrolet ending up in the tyre barriers.

Jack Harvey clipped Car #21 and then Kirkwood literally drove over the top of the #30 RLLR Honda given he too was unable to avoid becoming involved in the carnage.

Upon the Lap 50 restart, Dixon dived past Ericsson at Turn 4, which bottled the pack up again.

Power and Herta arrived side-by-side at Turn 8, where contact put the latter into the tyre barriers and brought about yet another Caution.

The Queenslander copped a penalty, sending Car #12 to the back for the restart which he therefore took in 15th position on Lap 55.

In the laps which followed, Grosjean stuck with McLaughlin despite the tyre compound disparity, although the leader may well have been conserving his softer rubber.

Immediately upon the critical lap, with 35 to go, Palou pitted from seventh, and then Ganassi team-mate Dixon from fourth next time around.

Grosjean continued to tail McLaughlin until he peeled off on Lap 70 and Penske responded by bringing the leader in on Lap 71.

That precipitated the race-changing incident and a fifth Caution, during which O’Ward filtered to the lead, from Ericsson, Dixon, Palou, Newgarden, Rossi, Ilott, Rahal, Lundgaard, and Agustin Canapino (#78 JHR Chevrolet).

Power sat 11th having been among those caught out by the timing of the yellow, but was first of the drivers on alternate tyres for the run home.

Palou and Newgarden both ran wide at Turn 13 as the field took the restart with 21 laps to go, while Dixon attacked team-mate Ericsson for second.

It allowed O’Ward to bolt to 2.7s clear in a single green flag lap, while Power was eighth when he passed Newgarden on Lap 82.

Meanwhile, Ericsson had shaken off Dixon and was half a second away from the lead with just nine laps to go, with still had more than 20 seconds extra push-to-pass in hand.

He had barely started to use it up when O’Ward was extremely slow off Turn 14 on Lap 97 due to a reported “plenum event” and Ericsson blazed past.

The Swede would not be headed thereafter, while O’Ward salvaged second place.

McLaughlin ended up 13th but team-mate Newgarden was not running at the finish after pitting due to a late fire in the left-rear corner of Car #2.

Ericsson will take the series lead to Texas Motor Speedway on April 1-2 (local time).

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Results: Race, Streets of St Petersburg

Race winner: 100 laps

Series points