Piastri missed the cut at the end of Qualifying 1 after running wide at Turn 12 on what should have been his final flying lap.
The mistake essentially forced him to run again, but with the best already gone from his tyres, the subsequent lap simply wasn’t enough.
It saw him one of two high-profile eliminations from the first part of the three-part session, joined by Red Bull Racing’s Sergio Perez.
“We didn’t have to but it felt like a very sensible decision to do that,” Piastri said of staying on track for a second push lap rather than boxing for new tyres.
“I just had very, very little grip on that second lap, which was a shame – but, I mean, I shouldn’t have even needed it.
“Obviously frustrating,” he added.
“The pace looks very strong and I felt comfortable, just tried a bit too much in Turn 12 and it bit me hard.”
Piastri had topped Free Practice 3 earlier in the day as McLaren looked to have the measure of Ferrari.
It followed on from a promising if underwhelming Friday that saw teams hamstrung by tyre testing in Free Practice 2.
“FP3 was very strong, even FP2. FP1 I struggled a bit, but FP2 I think we made some really good changes and I was comfortable,” Piastri reflected.
“FP3, very comfortable as well, and even in qualifying it felt good, just a very silly mistake trying a bit too hard in Q1.
“That was it. I mean, at least I know where it all went wrong, but it still is painful.”
Piastri estimates he lost a second with his Turn 12 off, which saw him ride the exit kerb on the right-hander entering the stadium.
Had he delivered that lap, he’d have easily progressed, with the estimated 1:16.6s on par with what team-mate Lando Norris managed.
Instead, he ran wide, the lap was deleted, and he’ll line up for Sunday’s race from 17th.
With the car pace in the McLaren, there is still hope of a good result, with Norris racing to fifth last year after qualifying 18th.
“We try to draw inspiration from Lando’s race last year and see what we can do,” Piastri admitted.
“I think it’s going to be tough because the field looks reasonably tight.
“But I think we’ve got some pace onboard this weekend, so try and use it to get through.”
The Mexico City Grand Prix begins at 07:00 AEDT on Monday.
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