Iannone won out in his late scrap with Petrucci for Friday bragging rights, the Suzuki rider pulling a tenth clear of the 2019 factory Ducati man with his final lap of the session.
Crutchlow, however, did not see the end of the session after being transferred to the medical centre via ambulance due to a heavy shunt at Turn 1 with 12 minutes to go.
Iannone took just five minutes to start knocking on the door of the fastest FP1 times, using a hard rear tyre to put his Suzuki top with a 1m20.256s.
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Andrea Dovizioso and Marc Marquez then both chimed in with times in the 1m30s, the Ducati rider with a 0.02s advantage over his Honda rival.
A little under 10 minutes in Marquez improved to set a new benchmark for the weekend, as his 1m29.739s eclipsed Maverick Vinales' FP1 effort.
The change at the top was followed by the first crash of the session, Alvaro Bautista dropping the front of his Ducati at Siberia.
Minutes later Marquez had his own drama at the same Turn 6 left-hander, a loose back end as he hit the brakes forcing him to abort the corner and straighline through the gravel. Shortly after, Valentino Rossi did exactly the same thing.
As the session neared the quarter-hour mark Crutchlow went quickest with a 1m29.616s, before Iannone edged that time by just over a tenth on his next run with a 1m29.510s.
But any chance of Crutchlow improving disappeared with 12 minutes to go when the LCR Honda rider tumbled at high speed at the first corner.
To add to LCR's woes, Takaaki Nakagami fell heavily at Siberia just moments later.
With five minutes to go Dovizioso made his move on a soft rear, leaping from fourth to second with a 1m29.555s before going quickest with a 1m29.406s.
That sparked a brief flurry of fast laps, Vinales going fastest before being immediately replaced by Petrucci's 1m29.291s.
But a final flyer proved to be enough for Iannone, as the Suzuki rider charged back into top spot with a 1m29.131s right at the flag.
That bumped Petrucci back to second, followed by Vinales and Dovizioso. Crutchlow was shuffled back to fifth thanks to his shunt, followed by Johann Zarco and Marquez.
Jack Miller was eighth-quickest, Alex Rins ninth, while Rossi the final rider currently sitting in an automatic Q2 spot.
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