Juan Pablo Montoya edged out Penske team-mate Will Power to claim the top spot in a rain-delayed second IndyCar practice session at Iowa Speedway.
Intermittent showers prompted delays throughout Friday afternoon, and also led to a handful of brief cautions during the session.
But last weekend's Pocono race winner Montoya made the most of a late spell of greens to trade places with Power at the top of the timing monitors before securing P1 with a 184.619mph average on his last lap.
Power's best lap was just a fraction slower at 184.584mph, while James Hinchcliffe and Takuma Sato both improved with their last flying laps to finish up third and fourth-fastest respectively.
Scott Dixon was Ganassi's highest-placed representative in fifth, but the team had been in charge during the early running.
Charlie Kimball enjoyed a brief spell at the top in the opening minutes before making way for Tony Kanaan, whose time remained the benchmark for nearly 45 minutes before Montoya stepped up.
Qualifying has been rescheduled for early Friday evening local time, with Power having drawn the opening slot in the running order.
Pos Driver Team/Engine Speed Time Gap 1. Juan Pablo Montoya Penske/Chevy 184.619 17.4327s 2. Will Power Penske/Chevy 184.584 17.4360s +0.0033s 3. James Hinchcliffe Andretti/Honda 184.421 17.4514s +0.0187s 4. Takuma Sato Foyt/Honda 183.860 17.5046s +0.0719s 5. Scott Dixon Ganassi/Chevy 183.616 17.5279s +0.0952s 6. Graham Rahal Rahal/Honda 183.503 17.5387s +0.1060s 7. Carlos Munoz Andretti/Honda 183.396 17.5489s +0.1162s 8. Tony Kanaan Ganassi/Chevy 183.152 17.5723s +0.1396s 9. Mikhail Aleshin Schmidt/Honda 182.598 17.6256s +0.1929s 10. Marco Andretti Andretti/Honda 182.071 17.6766s +0.2439s 11. Ryan Hunter-Reay Andretti/Honda 181.839 17.6992s +0.2665s 12. Josef Newgarden Fisher/Honda 181.646 17.7180s +0.2853s 13. Ed Carpenter Carpenter/Chevy 181.346 17.7473s +0.3146s 14. Ryan Briscoe Ganassi/Chevy 181.047 17.7766s +0.3439s 15. Charlie Kimball Ganassi/Chevy 180.526 17.8279s +0.3952s 16. Justin Wilson Coyne/Honda 180.083 17.8718s +0.4391s 17. Helio Castroneves Penske/Chevy 179.245 17.9553s +0.5226s 18. Sebastien Bourdais KV/Chevy 177.813 18.0999s +0.6672s 19. Carlos Huertas Coyne/Honda 177.427 18.1393s +0.7066s 20. Jack Hawksworth Herta/Honda 176.783 18.2054s +0.7727s 21. Sebastian Saavedra KV/Chevy 176.696 18.2143s +0.7816s 22. Simon Pagenaud Schmidt/Honda 175.681 18.3196s +0.8869s All drivers use Dallara chassis