With McLaren teammate and championship leader Lando Norris on pole for the third race in a row, the Australian needs to find something special on Saturday to avoid handing Norris a huge advantage.
The Briton is already 24 points clear at the top with only two rounds, and a maximum 58 points, remaining after Las Vegas.
Norris has out-qualified Piastri in their last three races, and beaten him in the last six, but the 24-year-old was hopeful he could at least limit the damage in the Nevada night race.
"There were a few things at the start of the lap that didn't go great from an operational point of view," said Piastri, who has qualified outside the top three in his last four races.
"I don't know how much time there was in there ultimately.
"But what I can say is there was more out there that we didn't get to use," said the disappointed Australian, whose last win was in the Netherlands at the end of August.
"We've got a good car underneath us that seems to be working well in all kinds of conditions. I think we can have a strong race tomorrow and hopefully make up some spots."
Team boss Andrea Stella said Piastri could probably have qualified third, behind Red Bull's reigning champion Max Verstappen without a yellow flag on his final lap.
"A little bit of a shame with Oscar that found a yellow flag in the final lap which would have been (his) fastest," said the Italian.
"P3 was possible with Oscar ... P5 is still a good position. I think we have a competitive car this year.
"Vegas has been one of the places in which we have traditionally struggled in the previous two editions so there has been an important investment in terms of understanding the technical and driving characteristics.
"We will try and do our best and finish on the podium with both cars."