The Australian ended the day fifth fastest with a best of 1:31.474s, a time that proved 0.006s slower than Yuki Tsunoda in the impressive RB.
But more concerning was the deficit to team-mate Lando Norris, who topped the times, with a 1:30.727s – seven-tenths faster than Piastri.
“Just not very comfortable with it at the moment for whatever reason,” the 23-year-old Australian confessed.
“I mean, I don’t feel my lap was great, but I’ve not felt that any of the laps have been great.
“Just need to try and find a little bit more, try and get a bit more comfortable with the car is probably the first thing, and try and find the limit from there.
“Our pace looks not too bad, we’re still somewhere towards the front, but obviously the gap to Lando is bigger than what it should be.”
Norris topped Free Practice 2 by 0.058s over Charles Leclerc, having trailed the Ferrari earlier in the day by 0.076s.
Piastri’s day wasn’t helped by a slightly delayed to start to the opening hour of running.
During pit stop practice earlier in the day the squad discovered an issue with the rear corner of the Australian’s car.
“During this morning’s pit stop practice we encountered an issue on the rear-left corner of Oscar’s car, which we are currently replacing,” the team revealed just prior to Free Practice 1.
“This isn’t an issue we’ve seen before, but we want to assess the issue fully to ensure we have no further issues this weekend.”
While repairs were quickly carried out, it meant Piastri sat out the opening 10 minutes of the session.
However, he was quick to point out that it was not the reason for his comparative lack of pace.
“It wasn’t an ideal start to the day but I don’t think it’s really impacted much,” he reasoned.
“It’s been a difficult day, definitely but I don’t think it was down to that.
“I just struggled to find a rhythm, really.
“The pace in the car looked strong, just not been able to unlock it that well.
“There’s definitely some things to go into tonight.”
Piastri remained among the leading contenders, and well clear of Max Verstappen who was only 15th after a horrid day for the Dutchman.
With McLaren looking to consolidate its place at the head of the constructors’ championship, and Norris chasing the drivers’ championship against Verstappen, it’s a welcome development regardless of Piastri’s deficit.
A final hour of practice remains ahead of qualifying for Sunday’s race.
Free Practice 3 begins at 17:30 local time (19:30 AEST) this evening.