Piastri will start tomorrow’s race from eighth, two spots back from team-mate Lando Norris with Yuki Tsunoda’s RB sandwiched in between them.
Heading into the qualifying hour, McLaren looked in the mix with Ferrari and Mercedes for top spot.
But while there were promising signs through the first two segments, Piastri was unable to make the gains he expected in Qualifying 3.
“Q3 we were nowhere and I don’t fully get why yet,” he admitted.
“The first run was on a used set of tyres, but that wasn’t a problem in Q2, but we really struggled [in Q3].
“Then the new set at the end, I just about managed the lap time I did in Q2. Everyone else found that extra step and we couldn’t.”
Across the three-part session, Piastri improved by just over four-tenths, his Q3 lap actually 0.01s slower than his Q2 effort.
Elsewhere, pole-sitter George Russell found almost nine-tenths, with around half of that coming in Qualifying 3 and Pierre Gasly, who qualified third, found a second across the session.
McLaren went its own way in terms of tyre preparation, a strategy that worked in the first two segments but curiously failed to deliver at the death.
“The first two parts of qualifying, we were doing something a bit different to everybody else, but it was working for us,” Piastri explained.
“Then, Q3, more or less we kept doing the same thing, and we just didn’t find any extra pace.”
Though starting eighth, not all is lost as overtaking around the Vegas Strip circuit is possible.
Piastri displayed promising race pace on Friday and the McLaren is expected to have sufficient car pace to clear both Yuki Tsunoda and Gasly in normal circumstances.
Furthermore, while Mercedes proved rapid on a single lap, there are question marks about its long runs, suggesting Russell could slip backwards, too.
That’s enough to leave Piastri to suggest there are “opportunities” available in the race.
“Race pace I’m a bit more optimistic,” he said.
“My long run yesterday was good, but for everyone it was only six or seven laps, and we’re probably going to have to go a lot longer than that on a medium tomorrow, which is going to be tough.
“But we’ll just have to wait and see whether we can manage the tyres the way we want.”
The fly in the McLaren ointment is Ferrari’s Carlos Sainz second on the grid and Charles Leclerc fourth.
The Scuderia looked the most competitive on race pace earlier in the weekend leaving it in a strong position to make inroads on McLaren’s constructors’ championship.
Ferrari headed into the Vegas weekend as favourite for the race, and that position hasn’t changed despite Russell snagging pole position.
McLaren sits just 36 points clear of the Scuderia heading into the Saturday night encounter.
“Until Q3 we’ve been pretty happy with how we’ve been relative to Ferrari,” Piastri confessed.
“It’s just we didn’t find that extra step.
“But yes, this is probably the track that we expect them to be quickest on, but we can’t just take for granted.
“We can’t just say this will be our worst and the next two will be much better for us, because if they’re not, then that’s not good news.
“On paper, when you look at the average, yes, we should be a bit better in the next two, but I don’t think it’s a given, that’s for sure.”
The Las Vegas Grand Prix begins at 17:00 AEDT on Sunday.
