Lewis Hamilton bemoaned a "pretty terrible" Japanese Grand Prix as he dropped away off the podium positions in the latter stages of Sunday's race to finish sixth.
Hamilton benefitted from a mid-race Safety Car and overtook George Russell on the restart to go into third but was unable to hold off the Mercedes and his Ferrari team-mate Charles Leclerc.
Lando Norris then overtook Hamilton late on as the seven-time world champion struggled for pace, seeing the chequered flag in the same sixth place he started.
"Pretty terrible ultimately because I was P3 and I ended up going backwards. I just need to understand where I was losing all the power," he told Your Site F1.
"I had a real lack of power, particularly in the second stint, but even at the beginning I couldn't keep up with people just through a lack of power.
"I don't understand this. I'm full gas, managing where I'm asked to manage and for some reason just lacking power, so I need to figure out if there's something wrong with the car or not. Still, we got some points."
It was Hamilton's trickiest weekend so far after finishing fourth in Australia and claiming his first Ferrari podium at the Chinese Grand Prix.
But, the 41-year-old trailed team-mate Leclerc throughout the weekend in Suzuka and was left confused about his lack of pace.
"I love being in Japan, I've generally enjoyed driving. It's just, as I said, I just really got trying to understand," he said.
"I had a really good stint of managing the tyres and then just didn't have the pace to just keep up. And it's never fun when you're just barely holding onto a pack. And when I got the fresh tyres, I still couldn't.
"I just couldn't. Just power-wise, I couldn't stay ahead of people, which was really confusing. I need to understand."
Leclerc bounced back from losing out during the Safety Car by coming out on top in thrilling battles against Russell and Hamilton to ensure he remains third in the Drivers' Championship, albeit 23 points behind Kimi Antonelli.
Even with the battling, Leclerc finished less than two seconds behind second-placed Piastri, who took his first podium of the season.
"Second place was maybe on the cards but I think we extracted the maximum we could," said Leclerc.
"With the Safety Car we were unlucky, Oscar had a bit of air to breathe and maybe he was a bit nicer with his tyres. I had to push straight away, which was a little tricky.
"I don't regret much. I think it was still a good race. Are we as fast as the Mercedes? I don't think so. They still have a big advantage. It's up to us to try and change that situation."
He added: "The Mercedes power units have a big advantage over us at the moment. This is a focus but we must not forget that there are huge gains in developing the chassis, the aerodynamics, putting the tyres in the right window.
"The engine we can't change anyway but in the mean time we need to improve everything around the car."
F1 returns on May 1-3 with the Miami Grand Prix, the season's second Sprint weekend, live on Your Site F1.
