Perez will line up alongside Charles Leclerc on the front row of the grid for the Belgian Grand Prix.
He qualified third fastest but moves up a place courtesy of a 10-place grid penalty for his Red Bull Racing team-mate Max Verstappen.
Lando Norris is the highest placed of the two McLaren drivers in fourth, with Piastri one spot further back.
The pair are expected to move forward in Sunday’s race and encounter Perez on track.
“Obviously we’re in a fight with Red Bull in the constructors’, and Max is leading the world championship in the drivers’ standings,” Piastri observed.
“So of course I expect Checo [Perez] to put up a fight, I think even for his own sake.
“It’s not been the easiest run for him and I think of course, more than anyone, he probably wants a really good result for himself.
“So I think he’ll fight hard for it, but I don’t think any harder than he would if it was another situation.”
Perez is fighting to save his drive at Red Bull Racing.
A poor run of results since the Emilia Romagna Grand Prix has seen him contribute just 21 points to his team’s tally in seven races.
In that same period, Verstappen had added 129, for a total of 150 points in the constructors’ championship.
Norris and Piastri have combined to score 214 points for McLaren, which now sits second to Red Bull Racing by just 51 points.
Perez will share the front row of the race with Charles Leclerc, the Ferrari driver banking a lap late in qualifying to steal an unlikely pole position.
Lewis Hamilton will join Norris on the second row, with the Brit confident McLaren has the pace to clear the trio of cars he starts behind.
“I wouldn’t say a concern,” Norris said of the potential for Perez to prove a roadblock.
“For our race against Max, we have to get past all of them.
“I know Checo potentially will have a bit more of a defensive mindset than the others, but Charles is still fighting off pole and for a win, so he’s going to be fighting hard for it – he’s not just going to let things go easily.
“Same with Hamilton. He’s always going to be aggressive and defensive when he needs to be.
Norris and Piastri topped Free Practice 2 on Friday, held in dry conditions, giving them confidence heading into the race – which is also expected to be dry.
“With the degradation this weekend, I think in practice, I pitted at one point in practice and I was three seconds a lap faster than the car ahead of me,” Piastri began, suggesting strategy will play a key part in how the race unfolds.
“If you manage to defend a car behind that’s three seconds quicker than you, then that’s pretty impressive.
“I think it will work itself out naturally.”
It’s a view Norris shares, suggesting the McLaren is kinder to its tyres than the Ferrari, Mercedes, and Red Bull Racing, though suggests the inherent pace of the latter will make it more challenging to clear.
“I wouldn’t say Perez is any more of a threat from that side, but he’s in the quickest car,” he observed.
“So in terms of getting past that, making the tyres last better than him and that kind of thing is going to be harder than some of the others.
“But I think when we look back at the pace from Friday, it gives us a decent amount of confidence that, if we just keep our heads down and focus on ourselves, we can hopefully get through them at some point in the race.”
While he expects to be able to get the better of Perez, Norris remains weary of the threat Verstappen poses.
The Dutchman will start 11th after a new internal combustion engine was added to his power unit pool, dropping him 10 places on the grid.
“Definitely a threat,” Norris observed of the championship leader.
“I think [he’s] clearly been quickest all weekend – even yesterday in FP2, they just didn’t turn up the engine and we had.
“So considering we were only two-tenths ahead, had a lot more in their bag, basically.
“Even after a lot of great races in the past few weeks, Red Bull have always been there or thereabouts.
“People want to try and count them out at times, but Max was easily on par with us in quali last weekend, and still pretty much the same race pace, just made more mistakes.
“At the minute, they look very strong here, he had looked [strong] since the first lap in FP1,” he added.
“He was quite easily the strongest, so he’s going to be a threat tomorrow.
“That’s low fuel, and hopefully high fuel is a little bit more even.
“But 100 percent, he’s going to be coming through quickly, and I’m sure he’ll be a threat for us at some point.”
The Belgian Grand Prix begins at 15:00 local time (23:00 AEST).
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