Piastri and his McLaren team-mate Lando Norris demonstrated impressive pace throughout Sprint Qualifying.
The pair ended SQ2 first and second before their pace evaporated in SQ3.
Rules dictate that drivers must use all three tyre compounds during Sprint Qualifying; hards in the first segment, then mediums in SQ2, and finally softs.
With the switch to soft tyres for SQ3, the pace Piastri and Norris had enjoyed evaporated.
Indeed, most struggled for grip on the red-walled tyres with pole sitter Max Verstappen admitting he was surprised his lap netted him top spot.
“I think it was just a really, really tricky session for everybody,” Piastri said of SQ3.
“I don’t know if the track changed a little bit or if we all just thought the soft was going to be a lot better but my lap felt pretty terrible and it was P6.
“So I think everyone must have just had a bad run – I think Lando did the quickest lap of the whole qualifying on a medium in SQ2, so a bit strange but I’ll take P6.”
Piastri starts two places back from Daniel Ricciardo, who was in impressive form for RB and offered similar feedback on track conditions in SQ3.
Meanwhile, both Mercedes missed the top 10, while Lance Stroll and Fernando Alonso will line up seventh and eighth, respectively, for Aston Martin.
Nico Hulkenberg in 10th is also well up for Saturday’s 13-lap hit out.
“It’s going to be tough for everybody,” Piastri reasoned of how the tyres will cope with the short-form race.
“The tyres seem to be surviving okay; they’re just very hot.
“So [I’m] not too concerned, but I think it could be an exciting Sprint.
“We’ve got a few cars out of position. I felt like nobody’s really had much consistency, so it will be exciting tomorrow.”
While the Australian will start the Sprint sixth, team-mate Lando Norris will line up ninth despite having the full McLaren upgrade.
“I just pushed too hard, simple as that,” Norris said of his SQ3 lap.
“The car was feeling very good. Just silly to be honest, couple of mistakes in Turn 1 and just a big spiral from there.
“So a shame, because the team has done a good job, the upgrades were working so I’m happy with everything, just not with one thing.”
The Sprint in Miami begins at noon local time on Saturday (Sunday, 02:00 AEST).