Norris was stripped of a third place finish after going off the track and gaining an advantage as he tried to pass Red Bull’s Max Verstappen.
Norris was ultimately classified fourth ahead of his papaya stablemate.
Despite Norris qualifying on pole position, Piastri said there was little surprise in the outcome of the race as Ferrari sealed its first one-two in the United States Grand Prix since 2006.
“Today seemed a little bit more positive than yesterday,” said Piastri, who finished 10th in the sprint.
“My pace relative to Lando at least was pretty similar for a lot of it. Ultimately we finished fourth and fifth or third and fifth.
“To be honest, it’s not a complete surprise where we ended up and where Ferrari ended up.
“Lando made us look more competitive than we were yesterday. Fourth and fifth today is not completely unexpected.”
Norris started from pole position but was boxed out by Verstappen at Turn 1, allowing the Ferrari duo of Charles Leclerc and Carlos Sainz through.
Norris dropped to fourth ahead of Piastri and held station until the first round of pit stops.
“Even the first two or three laps, both of us, it looked like we were really struggling to hold on to the pack ahead,” said Piastri.
“The first 10 or 15 laps we really struggled, don’t really know why. We were struggling a bit with the tyres and then they came back and the medium looked very strong considering it was pretty dead. A bit of a confusing one to read.
“Even the pace in the second part of the race was maybe not amazing but not terrible. Some things to try and understand. Obviously we can’t change the car from lap 15 to the rest of the laps. We need to understand why we suddenly found some pace.”
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Ultimately, what worked over one lap in qualifying didn’t over the duration of the 58-lap grand prix.
For Piastri, he hopes his fifth place finish is the worst result of the six-race swing to end the season.
“The car has been very tricky all weekend. Over one lap it’s had potential to be quick but trying to unlock it has been incredibly tough,” Piastri explained.
“And then over the race you can’t get away with the same things over one lap and the true pace really shows itself.
“I hope this is our weakest of the next six. We had some good races at the tracks coming up last year. We had some bad races at the tracks coming up.
“I think in general we should be a bit more competitive but I’m certainly not expecting the Ferraris to be out of the picture for the rest of the year either.
“They’ve been a bit under the radar the last few races. Monza they were obviously quick. Baku was very, very tough to hold them back. Singapore, without their poor qualifying, they would have been very, very competitive and this weekend is the same.
“This is not a completely unexpected result for them but we also need to try make sure we turn that around next week.”