Piastri, who's had a torrid time to start 2026 in his McLaren, didn't get to the starting grid in Melbourne and Shanghai but burst out of the blocks for Sunday's Japanese Grand Prix.
The 24-year-old Australian went from third to first before the opening corner, passing pole sitter Kimi Antonelli.
"Turns out we're alright once we start," Piastri said.
After a tussle with Melbourne GP winner George Russell on lap nine, which he survived, Piastri went from leader to sixth after a pit stop on lap 19.
He wasn't able to recover and Mercedes' 19-year-old Italian Antonelli won his second consecutive race.
Charles Leclerc of Ferrari was third, Russell fourth and McLaren's Lando Norris fifth.
Antonelli was leading on lap 22 when Oliver Bearman lost control and hit a tire barrier, triggering the safety car.
The Brit limped out of his Haas but was reported later to be in good shape by medical officials.
Antonelli said he got a bit "lucky" with the deployment of the safety car and it "definitely made life a lot easier."
Piastri also wondered what might have been, but acknowledged Mercedes probably had too much pace.
"It's a shame we never got to see what would have happened, but for us at this point to be disappointed about finishing second — is a pretty good place to be," he summarised.
"Massive thanks to the team, I think we did a pretty good job of executing with what we had.
"Clearly we still need to find a bit of performance but we took every opportunity we had today."
Antonelli won the first F1 race of his career two weeks ago in China, the second-youngest winner in history - only behind Max Verstappen in 2016 when only 18.
Russell was second in China a fortnight ago and won the season-opening race at Albert Park, which means Mercedes have victories in the first three races of the year.
Antonelli now leads the standings with 72 points.
"It's too early to think about the championship, but we're in a good way," he said.
"I got a terrible start, I just need to check what happened."
Formula One now takes a five-week break with races for April in Bahrain and Saudi Arabia called off because of the war in Iran. The next race is May 3 in Miami.
Antonelli's and Mercedes again showed they are best at mastering the new car configuration for 2026, which features a 50-50 split between internal combustion power and electrical battery power.
The cars are also lighter, narrower and shorter than last season with many drivers complaining about the most radical changes in a decade.
With AP.
