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Yamamoto, who left Dandelion Racing after winning last year's title to join Satoru Nakajima's eponymous team, suffered a difficult two days of testing earlier this month at Suzuka, where he ended up only 10th fastest on the second day of running.
Crucially, he also trailed new teammate Toshiki Oyu by some three tenths, and the gap between the pair widened during Tuesday's opening day of test at Fuji Speedway, topped by Oyu.
Yamamoto was eighth-fastest, but almost nine tenths off the pace in the afternoon session.
Speaking after the test, Yamamoto said that Oyu's performance laid bare just how much work lies ahead to have a chance of earning a fourth title in Japan's premier single-seater series.
Yamamoto told reporters at the end of the day: "We looked back [at the Suzuka test] and changed what we could, and we troubleshooted as much as we could, but I’m still not in my usual shape.
"I don’t understand why I’ve not been able to perform well, and when you see how well my teammate Oyu is performing, it’s clear it’s not the team’s problem.
"Honestly, I’m suffering from bottled-up emotions. I’m hoping somehow I can do more myself, but we’re not there yet."
Yamamoto said he could take little solace in ending the first day of Fuji testing slightly higher up the timesheets than he had managed to be at Suzuka.
"Basically, the problems from the Suzuka test haven’t been fixed," continued the three-time champion. "The problems I’m feeling are the same. The gap to Oyu is basically the same as Suzuka, so the situation isn’t good at the moment.
"I think what makes us look better in the rankings is that not many teams are preparing specifically for Fuji and we just happen to be ahead of some of those drivers."
When asked if he would consider this the worst start to a season he's ever endured, Yamamoto responded: "It’s not the worst pre-season I’ve had. It’s just not been a good one."
Yamamoto enjoyed a much smoother start to his relationship with Dandelion, where he moved in 2019 after winning his second title with Team Mugen, topping the first day of the second pre-season test at Fuji that year.
But despite his early difficulties with Nakajima, the 32-year-old remains convinced that the team has the potential to allow him to return to the front of the field.
"Even though the laptimes are bad, in the sense that we are ruling out different possibilities one at a time, I feel we are making progress," Yamamoto said. "It may take some time, but I think the team has the potential to recover the performance.
"If we can steadily troubleshoot the bad points and get back to our original performance level, I definitely want to be able to run at the front in this car. I’ll do my best to make that happen."
Naoki Yamamoto（TCS NAKAJIMA RACING）
Photo by: Masahide Kamio
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