The Australian now trails McLaren teammate and championship leader Lando Norris by nine points in the overall standings, with three rounds remaining after Brazil, having been only a point behind before the Briton won the sprint.
In a further blow, Norris secured pole position for Sunday.
"I've still got the belief that I can go out and win races and win the championship," Piastri told reporters after qualifying.
"But, you know, obviously things are not coming as easy as I'd like at the moment."
The 24-year-old has won seven races so far this season to Norris's six, but has not been on the podium since the Italian Grand Prix in September.
Piastri has also been beaten in the last five races by his teammate.
Norris capitalised on his teammate's recent struggles and retook the overall lead with a commanding win at the Mexican Grand Prix last month.
"The qualifying session itself wasn't easy, a bit of a bizarre session and weekend with the soft tyre just not working for some reason," Piastri said of his day.
The McLaren driver added that he struggled to improve his lap time through Saturday's qualifying and was unable to get the most out of his car, describing conditions at Interlagos as "a bit tricky".
Asked how he could turn things around on Sunday, Piastri said: "Just try and pass a few cars. That's all I can try and do, just try and capitalise on any opportunities that come up and see what happens."
The Australian will line up behind Norris, Mercedes's Kimi Antonelli and Ferrari's Charles Leclerc.
Team boss Andrea Stella said Piastri had been fast all weekend and classified Saturday as "just an episode".
"He has a very functional mind. The most important point is the speed is there," he noted.