Crashes, fires as Piastri fastest in chaotic second Japan GP practice
Two of the red flags were for trackside fires during a badly disrupted afternoon session in which Australia's Piastri clocked a fastest lap of 1min 28.114sec.
He pipped teammate and championship leader Lando Norris by 0.049sec in dry, sunny conditions at Suzuka, where home hero Yuki Tsunoda showed promise on his Red Bull debut.
Piastri and Norris -- who went quickest in first practice -- have won one grand prix each in a dominant start to the season from McLaren.
They had to keep their wits about them in an eventful second session that saw Australian rookie Doohan badly damage his Alpine in a heavy smash into the barriers.
There was further drama when Spaniard Fernando Alonso beached his Aston Martin in the gravel and two fires broke out on the grass around the track.
RB's Isack Hadjar was third fastest, 0.404sec behind Piastri, followed by Ferrari's Lewis Hamilton and RB's Liam Lawson.
Mercedes' George Russell was sixth, followed by Ferrari's Charles Leclerc and Red Bull's four-time world champion Max Verstappen.
His new teammate Tsunoda was a lowly 18th, 2.511sec behind Piastri, but without completing a representative lap on soft tyres. He was a promising sixth in the morning.
A highly eventful second practice was red-flagged about 10 minutes in when the 22-year-old Doohan spun off the track and smashed into the barriers at high speed.
The car was badly damaged, with one wheel flying off upon impact.
Doohan quickly confirmed he was unhurt and climbed out of the cockpit unaided before being helped back to the pits.
"Following precautionary medical checks, we can confirm that Jack is ok. Now we're working hard to get the car ready for tomorrow," Alpine said on social media.
The session was halted for around 20 minutes and it restarted for only a matter of minutes before it was stopped again.
Alonso barrelled off the track and came to a halt deep into the gravel.
The Spaniard radioed to say that he was stuck and his car had to be lifted away before practice could continue.
The red flag came out a third time when sparks from the cars started a small fire on the grass surrounding the track.
There was just enough time for another stoppage with 10 seconds of the session remaining when sparks started another blaze.
The session marked an eventful first day behind the wheel of the Red Bull for Japan's Tsunoda in his home grand prix.
The 24-year-old was catapulted into a Red Bull seat last week when Lawson was axed just two races into the season.
Norris went fastest in the first practice session ahead of Russell and Leclerc.