Final practice got underway at Road America with the rain falling hard.
Unsurprisingly, there was moments immediately. Kyle Kirkwood and Scott McLaughlin both ended up in the tarmac run off at turn 8 as they had snaps of oversteer in treacherous conditions.
At turn 5, Colton Herta locked up and went straight on but continued without issues. Ferrucci also got a deep at the same turn.
There was another moment for McLaughlin at turn 3 as he ran wide, ended up on the grass and took a trip through the gravel before carrying on. The Kiwi continued to have difficulties as he had another off course excursion at turn 5.
The biggest moment in the opening 10 minutes came from Christian Lundgaard as he aquaplaned braking for Canada corner and went straight off at turn 12. Amazingly, he just avoided the barriers and reversed his way out of trouble.
Defending champion Alex Palou caused the first red flag as he got wide, ended up on the grass at turn 14 and got stuck in the gravel.
A 2:09.0104 from Colton Herta saw him almost 2 seconds clear of Christian Lundgaard before the session resumed. Rahal, Pourchaire, Palou, McLaughlin, Robb, Newgarden, Rasmussen, Ferrucci, Fittipaldi, Lundqvist, Armstrong, Simpson, Ericsson and Grosjean had also registered times.
A second red flag was thrown as Pietro Fittipaldi aquaplaned in the same spot as Christian Lundgaard, getting stuck in the gravel. Alexander Rossi almost followed him in moments later as the red appeared, with the AMR safety truck about to come out from behind the barrier at the same time. The 32-year-old got a 5-minute penalty for it.
Herta continued to dominate in horrendous conditions as a 2:07.3027 had him almost 2 seconds clear of Will Power. Lundgaard, Ericsson, Rahal, Pourchaire, VeeKay, Robb, Rossi and Fittipaldi completed the top 10 as the action got underway again.
The rain and track conditions got even worse, forcing the drivers to pit. After a 15-minute spell, the drivers started to venture out in the last 10 minutes as conditions got a little bit better.
Multiple drivers managed to improve. Christian Rasmussen moved up to P3, with Scott McLaughlin climbing into the top 5 as well.
A third red flag came out as Kyle Kirkwood lost the car at turn 14 and spun on to the grass on the inside before pit entry.
The green flag thrown with just over 6 minutes left but the red appeared again immediately as the #10 of Alex Palou came to a stop on pit exit as car gremlins struck.
Herta, Power, Rasmussen, Lundgaard, McLaughlin, Ericsson, Rahal, Rossi, Pourchaire and VeeKay was the top 10. 6.6s covered them.
