With his teammate, world champion Lando Norris, only finishing sixth on Saturday, the Australian admitted it was "going to be tough" for McLaren to battle out of the midfield as Max Verstappen looked the man to beat again following his first pole of the season.
It felt almost like the clock had been wound back with Verstappen and his old rival, Ferrari's Lewis Hamilton, back on the front row together, but McLaren looked out of the picture with Norris conceding the team was "struggling" at the Bahrain race that's been moved to Malaysia because of the Middle East conflict.
"It was tricky out there for sure," sighed Piastri, who'll start from sixth on the grid after third-placed qualifier Isack Hadjar suffered a five-place penalty for an engine change.
"I think my last lap I went through turn two and didn't have a whole lot of grip so I need to understand what happened there.
"Other than that it was a decent session, just unfortunate to not quite get that last lap as good as I wanted it to be. I thought it was a pretty good session in general, just we're not fast enough yet.
"It's going to be tough tomorrow. I think our long run pace did look pretty good yesterday, but the others around didn't look bad. We'll have to wait and see what we can do, but I think it's going to be a challenging afternoon."
While Verstappen was dominating, the McLaren duo were behind both Ferrari drivers and Mercedes' championship leader Kimi Antonelli, although the Italian's teammate and nearest challenger George Russell was dealt a title blow after qualifying only eighth.
Verstappen, though, looked back to his best on F1's Malaysian return as he grabbed his first pole since the final round in Abu Dhabi last December.
The four-time champ won the last race staged in Sepang and, nine years on, declared: "It is fantastic. We have been trying to get pole and to finally get it – from where we started the season – is pretty incredible."
